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.
People will find it hard to have fun anymore when they faced a bad result (lose) and unfortunately at such situation, people tend to forget their previous plans regarding gambling. People will keep chasing their previous lost instead of stopping at that point.