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.
I believe that a gambler that gets too addicted to gambling any circumstances might happen. It still not going to stop until it meets the end or in short, the end wherein you will realize its regrets. Also gambling can't be considered as way to repay loans and debt, it might also be the reason to worsen the deficit, also don't think that even a gambler wins big money will stop playing I guess its mindset will worsen only because he/she will believe that he/she has a lot of money and can't lose that much and eventually without realizing all the money will vanish in instant.