Honestly I don't know like most of the other gamblers,because we may win many times but still wanted to gamble so any amount can't make us to stop from betting but if someone really new to betting but they won in few millions then they might quit gambling for a reason.
May be the need of the people varies and some has some loan or debt to repay or require some extra money for something and some just gamble to pull out hat extra money from gambling, may be that would be one of the reason and that amount could stop them once they achieve it. But more likely greedy behaviour of a person would not prevent them to continue gambling.
Gambling is definitely not something which can help you to repay a loan. If someone is stupid to the extent that he/she starts gambling with that goal, I can guarantee that they would not stop until they lose everything, even if they initially won what they needed.