Ideally, you want subs long enough that camera read noise is negligible compared to shot noise in the background of your image. There's a description of a simple way to evaluate this here:
http://www.iceinspace.com.au/forum/s...d.php?t=117010
Unfortunately, unless you have a very fast optical system or a camera with very low read noise this will require impractically long exposure times with narrowband filters. So, expose for as long as you comfortably can. If you have good polar alignment and guiding then the limit will be how much time you are willing to lose if something goes wrong.
I do narrowband subs of 30 minutes and have done 60 minutes on some very dim targets.
The reason you can't see much difference between your 10, 15 and 20 minute subs is probably because you're not looking at the dimmer areas closely or stretching them hard enough. Bright parts of the image will reach high SNR quickly. It's the dim areas where noise will lurk.
Cheers,
Rick.