As long as you making +ev decisions, and the table isn't too tough, I'd say sit as long as you can play. Also,Don't fool yourself into standing up as soon as you get even. I like to never think of myself as stuck or up, what you have is what you have, if that makes sense. As long as you have the
bankroll, just focus on making +ev decisions and in the long run you make money. If your stuck, standing up to cut your losses may actually be cutting profit you could have made. It is important not to be results oriented, espically in cash games. Now if your stuck because the tables bad or your on tilt/not playing optimum poker, then yes you should stand up and try your luck another time.