It doesn't the scale for a gambler to know that it is the time to stop when others are already affected with your hobby or addiction in playing. On before it happened then you should be mindful with the things that it will happen if you did not stop your playing activity,
It is time to stop :
1) When your friends or family start complaining that you are not giving them proper attention and time because o this habit.
2) If you start ignoring your responsibilities towards parents or children because of this, it is the time to stop.
Your answer is for the people who wish to quit gambling for good, I think topic is more about how to stop when you are making profit and to leave gambling session richer then on start. I believe stopping with gambling session or quitting gambling for good have something common, its control over your actions. You need to have control over your emotions if you wish to stop with anything in any moment. Without control you are not able to make decisions that you will stick with, you will think one thing today, tomorrow you will think something else.
There are a lot of talking about this, but in the end individual experience is always unique, you need to know your self in order to have benefits from everything you do, including gambling as one of hard activities, where you can lose a lot of money if you don't know what you're doing.