I go until I get something foul tasting from my hookah(not just bad heat management) or 2 rounds of coals. My coals usually last me an hour and half - 2hrs. I never go more than 2 rounds of coal to my Egyptian bowls. I always repack and go again if I still feel like smoking.
I usually stop because I'm going to bed or going somewhere, but if I'm home 9/10 that hookah is lit.
Rarely do I stop in the middle of the bowl though. I Don't smoke if I know I'm going somewhere or it's too close to bed time. No reason to waste in my book
Hookah is relaxing to me. If I've got to hurry up and be somewhere, I'm not smoking until I'm able to devote time to it.