As the questions says, when do you stop? your point of no return?
a. at least double your money, then stop
I think, based on my experience, this is the best time to stop, of course, after you've enjoyed your session. If you still want to have it, at least save a portion for you to celebrate your win.
It's a great feeling to have fun and enjoy your session, but it's worth getting out and looking forward to celebrating your winnings, no matter how small they are.