when the party is o-v-e-r. Because you've run out of stuff or run out of money; because its Monday morning; because you're goin' down; because its Day 3; because everyone else left hours ago; because the wife/husband/g-f/b-f is pissed; because its interfering with your ability to have sex, etc etc etc.
Dude, its 6:45 a.m. and you have to be at work at 7! Put down the pipe, game over already!