I usually start with 6 tables, and close the ones that seem slow after a couple of minutes. I usually end up with 3 tables because it leaves a little space for firefox or a media player.
And I play until I loose focus, or I have to do something else, which is usually around the 2 hour mark. If I play longer that's where I make a break.
I don't set any monetary limits. I always have auto-rebuy-to-max on. I don't have to worry even if I go on a record loosing spree though, but I do usually quit early if I run bad simply because it screws with my head, and I can't help it. I don't really steam that bad, but I play worse... I tend to overcompensate and play too tight usually.
If I make a bet and someone calls that I didn't notice was in the hand, that's when I'm getting ready to leave. Or when I bet every street with TPNK, and after the hand I check the player notes and it sais "DANGER DANGER, UEBER ROCK!! Draw out on that sucker!"
avoidable mistakes.
More often than not I have more than 10$ on 3 5$ max tables after an hour, so carelessness can become expensive if I clash with another big stack. But I like playing deep so I rather quit than cash out and start from scratch.