When do you know it is time to leave the casino?  When that voice in your head says, "One more hand/spin/roll."  DON'T DO IT.  Just go.  That is when you know when to go. Because when that voice gets in your head, you'll lose.  Then you'll want to get that loss back. But you never will, and it will be all gone.  So when you think, one more time. That's the time, don't do that last bet, just leave and be happy that you got what you got.