When to call it a day?

[boudge7]

I 4 table the $30's, as my "summer job." My question is how often should I take a break, and when should I stop for the day. My schedule right now is, play about 12-15 break to eat, play 12-15 more. Take another break and maybe play a few more. Those of you who play many a day, do you set time limits or # of game limits.