Oh buddy you are about to open a can of worms.
https://www.youtube.com/watch?v=3RH93UvP358I recommend watching that! Your sub lengths are going to come down to your camera, it's settings, your focal ratio, and your sky conditions. These considerations are mostly related to your total stack noise and how to optimize your exposure times for that.
On one of your pictures I see you are in bortle 7, so maybe something like 60 to 120 seconds.