Stars exposure length

I think it is going to depend on:

1) magnitude of brightest stars in your field of view

2) F-ratio of your OTA

3) gain (High? Low? Unity?) for ease of processing it might as well be the same gain you shoot the rest of your lights. 

4) type of OTA (reflector vs refractor) and, if reflector, how much you want to try to minimize diffraction spikes…

Good info Tareq,
I will caveat my answer because I do not separately image my stars from my targets. I merely capture my stars in OSC or Narrowband along with my target, then I separate the stars from the target, color correct/process them separately, and recombine at the end. That way I avoid the magenta/weirdly colored stars and I manage to get the same integration time for my stars as for the rest of my target.

I mentioned magnitude because if I have a REALLY bright star in your field of view (think Betelgeuse, Rigel, or Sirius), I tend towards shorter subframes (maybe 10-15 seconds even) to try to preserve a little more color/reduce the diffraction spikes more, but you didn't seem to mind those... so you can probably disregard that.

That being said, if your desire is to shoot RGB/OSC stars and combine them with narrowband, given your setup, then I would suggest starting with 30-60sec exposures at "unity gain" which is about 100 with your IMX571. The IMX571 has a pretty large file size, so if your file storage/computer will bog down with two many subframes then maybe lean towards 60 sec... This will accomplish two things:

1)  Short exposures will minimize tracking errors and should give you rounder stars (assuming collimation/focus is tight).
2)  Short exposures will minimize the chance of overexposed star cores, preserving the different star colors

Given enough sub exposures, the actual individual exposures won't matter too much.

Now for total integration time.

A good "test subject" for what you want to do is globular and open clusters.  They are pretty much all stars so there are no more faint galactic arms or nebulosity to try to capture. You can concentrate solely on the stars. It is really easy to think that can just jam out about an hour of star subframes and call it good. If you look online you will see a lot of people who talk about just needing 2-3 hours of subframes even. Open clusters are all about not overexposing your star cores and getting your star colors down pat.

My personal opinion is that total integration time for stars should be as long as you can stand. If you can get 5 or 10 hours, that is great... 15-20 hours is even better (with diminishing returns), and this is why; 

1) There is noise in star data, and the more subframes you can average into your final image the crisper and tighter your stars will appear.
you can look at my M38 image (https://www.astrobin.com/full/yptq6g/0/) which is less than an hour of total integration. Zoom in and see what I'm talking about with star noise. It was a good study in stars for me, but I need to go back and add time in the future. Sidenote: this was made from 3 minute ASI2600MC Pro subframes at gain 0, so if you drop the gain, you get some well depth, and preserve color more easily, but now you are subject to more tracking errors and still have to clock in the total integration time... back on topic...

2) You will pick up more and fainter stars than you ever thought previously possible.

When I started focusing on M13 (https://www.astrobin.com/r2b3dy/C/) a few months ago, I initially thought I could make do with an hour of integration (original)... and I later added more integration time to get to over 5 hours. You can see the comparison... how many more stars come out after the additional time. My goal... if I ever have a clear night again in my life... is to get up to 10 hours. Theoretically, 2 hours is twice as good as an hour... 4 hours is twice as good as two... then eight... then sixteen... but at some point you are processing a LOT of subframes to get an incremental gain that you can't perceive with your eyes. Generally speaking 24-36 hours of subframes are somehow considered "enough" for heavy light pollution areas, but your mileage may vary and you may also be comfy with a third of that for stars alone. My skies are between Bortle 7 and 8, depending on the amount of dust in the air. With Bortle 9, you are probably close to needing double what I'm happy with... it just takes patience and time. With your double rig, if you are taking RGB data with one camera and narrowband with the other, it may work out great. I've personally never used a rig like that.

Honestly, I'd probably say 5-10 hours would be enough...to start... That is 300-600 60-second subframes and 600-1200 30-second subframes. That is a lot of storage and computer resources, so if those are limited, you can hedge the lower end of that... If you are patient and/or you can combine multiple years worth of data over time... you know your star data will only get better. Start with an hour of star data... then add 4 more hours and compare... then add 5 more hours and compare... did you notice a difference?

I'm sure others will recommend more... or less... an any will give you a result and more time will always give you a better result. 

Hope this helps? Is that what you are looking for?
The answer is always "it depends," but to my eyes, 5 hours is around the bare minimum.
You can dig into histogram analysis and signal to noise ratios... or go with the old WAG (wild ass guess) that has been a ballpark solution for me.

Also in future i have to think about my reflectors scopes ideas for multiple imaging setup, those are the main reflectors i have now for DSO:

1. 10" F/8 RC Truss
2. 8" F/5 Newt solid tube
3. 6" F/4 Newt solid tube

So if i will use those then i must try to find backups or pair to those to make things easier faster for me, let's say RC is mainly for galaxies and clusters so 90% broadbanding, if i use it as a main it means i have to find another scope cheap for Lum to add to RGB and Ha using mono camera, i think i might go with OSC over mono for RGB while my mono is for LSHO, many said that RGB is for coloring mainly and can go with shorter exposures, while Lum and SHO are holding the main strong details, so RGB mostly is to get colors and Lum will give the details, this is good idea.

Later i can think about another backups for 8" and 6" too, Newt are cheaper, RC is expensive and i was lucky to save and buy one, and in future if i buy a bigger mount to carry it then i better think about a second scope next to 10" RC but not another RC, not 10" or smaller or even larger, but the only scope i can think about is either 10" Newtonian or 12" Newtonian, in any case the stars will be taken from one scope so spikes are fine, the other backup scope is for either Lum or Ha or OIII or both [if i do planetary nebulae].

I wasted like 3-4 years with only one setup and always short time and issues here and there with setup, so when i get successful setup i end up with really short data, so it means i need to live forever to finish so so many targets in the sky every year, i hate to spend many or several nights only for 1 target, and being under Bortle 8/9 i know i always need like 5-50 hours, even that 5 hours sometimes i need like 2-3 nights to complete, imagine if i need like 2-4 hours per filter, this is too much, that is why i keep thinking about multiple setup, i already have several cameras and scopes and i can add few more later, so why not, i don't drive at all to darker skies, and i don't want to miss with so so many steps in processing mainly because of stars alone.

I understand your dilemma. I used to jump around from target to target as well. Now I sometimes spend a month on a target until I get at least 24 hours of integration to work with. 

it is frustrating because I can’t capture all the different targets that interest me, but if I have it my way, I’ll be pursuing this hobby for many many years to come, and I trust that I will get there eventually. Plus, the lessons I learn with processing deep integration times will only help me as I get more experienced and attempt more challenging targets. 

as I get older I find that patience makes up a lot for speed and enthusiasm. 

I personally think gain 100 is a good choice for you. Perhaps one final consideration, if you are open to the suggestion; maybe try RGB instead of LRGB. These days I really only use the L filter for focusing and plate solving. I have attempted Lum subframes many times but generally, if I have to shoot in heavy light pollution, my Lum subframes are less beneficial and tend to mess with my final color balance… most of my colors end up getting washed out even if I get a little more detail. The RGB filters each do a pretty good job of isolating out parts of the light pollution together. Now a 6 hour summertime window for image capture can be split evenly between R, G, and B (or SII, Ha, OIII) with 2 hours each… if you can manage that over four nights, that’s 24 hours of integration. 

while I have definitely used OSC, and I have an IMX571 color camera, I find it is difficult to successfully integrate OSC and RGB without some sort of pixel math, which isn’t really taking advantage of any of the noise reduction of multi subframe pixel averaging. Unless you have a solid gameplan (that works well) for combining OSC and mono data, I might recommend choosing one or the other and sticking with it… although if you are truly just using Mono RGB or OSC for stars… you can probably ignore this last paragraph. 

regardless, your setup sounds more complicated than anything I could successfully manage, so I wish you the best of luck and all the clear skies.

I have IMX571 color and IMX571 mono, so if i use that color for RGB then i use the mono for Lum and SHO, most likely targets are part of that, i mean either LRGB only or HaRGB or LHaRGB or SHO+RGB, so if i leave OSC for RGB alone then i can have all time with mono to do Lum and SHO, but i asked here for exposure of RGB so then i can decide for that third IMX571, should it be second color IMX571 or second mono IMX571, i also was planning to get that dual NB filter for color camera soi can image Ha and OIII together for whole time available, then i can have second color camera for RGB, and that leave the mono for SII, but if short exposure is enough for RGB then maybe another mono, but in this case i better buy a quad so i never worry about back focus and make sure it is a good Quad scope so i don't need to refocus each RGB filter.

I also have two 60mm doublets, both with old mono cameras of Panasonic 4/3 [QHY163M and ASI1600MM], so i might think for third 60mm in future but then which camera, color or mono again, or, i just pass the 60 and use my 72mm [FRA400] with its QHY294Mto be main scope for Ha and leave two 60s for OIII and SII, i can think of so many options with my whole setup, i spent like 3 years only to collect gear instead of collecting photons, and later once i am done with gear then i will be busy with photons whole life i hope.

Sounds like a good plan. Clear skies!