when to leave?

I was wondering....

when do you leave the tables...
a. when you're tired?
b. when you've won a certain amount (what amount?)
c. when you lose a certain amount (what amount?)

do you limit yourself to time, or just go until you gotta leave?

I always find that i'm usually down at the end of the night and was thinking i should quit when i make a certain amount. bad idea huh?