<sigh> unfortunately I don't have a stop-loss. I think it really depends on your style. If you can bounce back from a loss of a couple buy-ins, then keep playing if the game is good. If you can't, then I think you need to leave after x buy-ins (if you don't have a track record of every coming back from being stuck 3+ BIs, what makes you think you can do it this time)?
One quirky thing I do is when I am done, I'm done. I don't wait for the blinds to get to me. (cue the "they're free hands, you've already paid for them" comments). It's not that I'm afraid to play AA the last hand of the night, it's more that mentally I've checked out of the game. If' I've checked out, then I'll play the hand poorly ... which costs me money.
The one exception I make is at Gemini if I paid for the bad beat, then I'll fold the free hands in case someone at the table hits it.