When do you end a session?

My main worry in my poker game is not knowing when to end a session.. A prime example of this is today, was playing a 2/5 and was up ~$500 after roughly 2hrs of play. I then continued playing but ran into some bad beats/challenges etc and bled chips for about an hour or so, making a small comeback to around $300 profit after the ~3.5hr mark. Then in the following 2hrs i lost the $300 profit and the original investment into the table (due to a large combination of things we all experience, ranging from bad beats to bad plays and ill timed aggressive moves etc.).

So my question is, When should you end a session?, and what factors should you take into account?.. Should you set a goal of money to reach then just leave? or a specific amount of time to play then leave? or just play it out till you can't play no more and see where you had ended up?..

Thanks.

Take plenty of factors into account.

Mainly how you feel the loss's are affecting you.

Because that's when you should take your break.

People are also money hungry once they see you've taken theres.

Keep this in mind and maybe tighten up a lot once you've cashed in a big win, you can still win more this way.

I stop if I lose 3 buy-ins, no matter how well I feel I am playing because I am likely playing poorly, have a bad image and need to maintain my bankroll.
Otherwise, I quit when I feel I am playing bad or I am becoming emotional.
If I am winning at a good rate I play as long as I can.

If I am winning I will play until I have to go or if I get back to even because of bad play on my part. If I am losing I will usually end a session when I am down 2-3 BIs. I might even change tables if I am down one BI but in PLO8 another table of the same type is rare to find at my level (micro) on Carbon

I play as long as fatigue does not affect my game. If i am not in the mood to play i do not play at all in one day.

I play until my opponents have no chips left.

thanks for the responses. The main consensus i feel im getting from this is that you should play till you HAVE to leave due to something affecting your game play. Otherwise play on and maximise profit to its fullest potential rather then trying to walk just because your up.
This seem right? if so i do like that thought process

When playing cash games, I'll have a pre-determined stop-loss and take-profit. ie. I will stop if I lose $X or make $Y. For SNG's I use table-ninja, which I can load in an amount of tournaments to register for in my current session, when those tournaments are finished, then my session is finished and I stop .

IMO, you shouldn't continue playing just because you're having an excellent run, because you will be kicking yourself once you start losing, as in the case you described. Also, you shouldn't continue playing just to win back your losses, that is a destructive pattern of a problem gambler.

Just set your goals, and be consistent.

Assuming I don't have a higher non-poker priority, I play as long as I feel I'm reasonably +EV. This can encompasses a number of factors. A few important ones are the opponents at my tables, my mood, my level of mental alertness and my level of fatigue. But just listing these is somewhat simplistic since they can involve various factors. For example, a particular table may be alright or not depending on where you're seated relative to the tough players and the weak ones.