The penis can survive for some time without blood flow. If you can do 20 minutes in one go that would probably be fine (cell death takes much longer). I could never stretch more than 5 minutes, lowering to 3 minutes as the session goes on. I never had any problems with that length of time.
I think manual stretching has a degree of variability to it that you might want to take three tries to get into the perfect stretch (comfortable and effective). Once you are there it makes little sense to drop it after 50 seconds.
This is a time/stress thing and the more time you put in the better. The tunica is harder to stretch so you need to be as effective as possible.