Everyone has a different system for how long to play, when to quit, etc.

There is an old poker saying that goes something like "As long as you have an edge in the game, you should keep playing". Basically what this means is that as long as there are enough players in the game that will donate money by playing poorly, then you should stay at the table. Now this is obviously not workable for everyone, especially those who have time commitments.

I suggest something along the lines of setting daily win goals, along with daily time goals. So, each day you want to play for 2 hours, quitting as soon as you hit 2. Or you could say I want to make $100 (or equivalent for your level, $5 NL, not realistic). Then no matter how long you've been playing, quit once you hit your goal. Or you can do a combination, I will play for 2 hours, or $100 in winnings, whatever comes first. Some players also set stop loss goals, so that they don't endure horrific losing days. I will play for 2 hours, $100 in profit, or 3 buyin losses, whatever comes first.

Try it out, and see what works for you, each player is different and no one system is superior to another.