I am fortunate enough that I can walk down the street from work and be at my local card room. I try to get a few sessions in each week. Typically try to keep my sessions to about 3-4 hours. Try to focus on the game conditions and hours played. I don't have a, "I am up $X its time to go." I was lucky enough today to get paid on a few hands and was up about 250bb at about the 2.5 hour mark. You guessed it, KK vs 22, against another big stack, I am now flat on the session and tilted. Decide it is time to go, don't want to play emotionally ect.
Sorry for the rant, I know its a cooler, move on. It isn't about losing the pot. It is about when I looked down at my stack and started adding it to my bankroll, was that my cue to leave?
1) You lose your stop loss limit
2) When the game dramatically changes and isn't as good/is going downhill (when whales leaves and are replaced by sharks)
3) When you feel a previously hand/action will dramatically effect your play, like a bad beat. Basically, if you're ultimo tilt and that tilt counter-balances the advantage from it being an easy table, the it's time to call your 'last orbit for me'
I hope this helped somewhat, I personally find knowing when to leave one of the hardest aspects of the game