If losing, when and why do you stop?

[Marlow]

Was cold-decked for about 1.5 hours last night. I lost a modest amount to a good opponent, but when I finally managed to win some back, I kept playing and managed to lose it all back and then some. At the very end I lost a buy-in out of sheer frustration.

Anyway, to answer my own question: I stop when I get back to even, I finally "figure out" a good opponent, or when he gets eventually gets up. None of these are good reasons, and are terrible leaks.

So I'm interested in how other people manage to leave the table when a session isn't going their way. Thanks!