TIME management advice? When is it time to leave?

Hope it's ok that I posted this in 2 diff. forums, to try to get a response.
http://beatboxworld.com/

I play live 1/2 NL at my local casino, usually buy in for $100 ($200 is the max). I'm an ok player, nothing amazing, but I find that if I stay in a game long enough, I always lose all my winnings, plus my original buy-in. Usually, it's a bad beat that does it. I am usually able to get between $80-200 profit before things start to go south....sometimes more, sometimes less.

For example- last night, I started playing at 1AM, turned $100 into $200 in about 3 hours (got no cards for the first coupla hours). The remaining 4 players were very wild and I figured I could build my stack even more. I flopped 2 pair and bet $40 and a dude called, hit runner runner for a flush....went all in, I called (stupidly)...and was done.

You can certainly argue I played that poorly, but my main question is- how do you know when it's time to leave? I have read somewhere that if you keep playing at a table, you will probably eventually lose everything. But I look around at some other players at my table and they've turned $100 or $200 into $800 or $900. For me, time management seems to be the bigger issue over bankroll management. I guess, at some point, you have to know when to get up and go....but I never know when that is.