Once we go beyond our limit, I think that's the right time to stop. When we're spending too much without earning any profit that is when to stop. When it's affecting our plans for the future and when we're addicted we should stop.
No, never go beyond limit. Time to stop is when you reach your limit, if you reach limit and go beyond it there is possibility that your next limit will be when you lose all money.
I know that for a very desperate person like me that wants to be rich, will only stop if I had no money left in my pocket. The urge of hitting a big amount of money will push ne to bet again for many last times until I had nothing left after losing.
You should never gamble if you need money desperately, it is one of the main gambling rules. Your story sounds like a problem, have you ever consider to ask for professional help? Please do.