When do you stop? Do you have a daily limit or something?
It is very simple mate, People should stop playing if they can not afford to loose more money. If you have enough money for your living and the loss in the gambling does not affect your personal life then you can continue but it is always advisable to play with smaller amounts.
When you start gambling with the purpose of having fun then it's easy to stop but when you are gambling for the purpose of making money and you are currently down then I'm sure it will be hard to stop and accept the loses then move on. Most gamblers who are in the situation will fight there purpose and they will not stop chasing for those loses to satisfy them, the sad thing is that when you are in the situation it's very hard to win anymore since you are not focus on your technique and you don't think realistically anymore. Emotional attachment is a big hindrance to success so we have to be strong and in control to avoid that.
That's true for people who are really just for having fun and I am like that towards gambling. Right now, I'm just enjoying how I gamble and keeping it with a steady pace. If you try to keep yourself comfortable and contented, you wouldn't have trouble stopping yourself with it.