When you no longer have fun when you play, just take a break to start fresh later. Gambling is a game that is meant to entertain us. If you see it as a business that can be profitable, you're doing it wrong. You should stop when it's starting to affect your financial situation.
For me, gambling is all about fun. Once you loose that, it's time to rest and a break can be good.
Oh yeah i definitely agree with you. We should gamble for fun, not for profit, so we should stop gambling asap if it get's stale and boring. Sometimes, even after you make break it's still boring, so it would be bad idea to gamble then as well.
Another good sign to stop is when you lost mos of your money. Person that did that just should stop trying to win back what they lost and stop gambling for a while. Else they would lose even more.