When to leave a table?

[Leo Bello]

I have an interesting question... I have been winning a good par of sessions at 1/2 but having one problem. Knowing when to stop. Normally I get to my profit early in the session, like in the first 20 minutes, then I begin to even up, losing some, and winning some.
In other situations when I start falling behind, I manage to catch up and make it even after some time (normally a long time, little by little, tightening my play the most I can).
The problem is this become long sessions, 2, 3 hours to:
one had an even or low profit sessions
or
have a session with winnings I made practically in the first 20 or 30 minutes.

(one of the reasons seems to be people start reading better when I tighten, so some good hands like KK and AA end with low pots, after all slow playing after a raise pre-flop to cut limpers, is something difficult in limit, unless u have nuts and let people do the betting).

Anyway, how do you guys feel it? Set a goal/target per table? Set a time per session? Let it vary?