If there is no agreed set time or number of sets to be played---quit any time you want. The question is "When is it ok to quit as a winner?" If the question is directed at me, then I answer it as expressing my opinion and quiting as a winner is best for me. Gambling until you lose is for losers. Since there is some confusion about this in general, I let it be known that I will quit when I want before we start.
Think of it this way. If my job pays $20/hr. and after working all week, tonight I'm out to play pool. Some one I don't know comes into the pool hall and wants to play 9ball. We agree to a race to 7 for $100. After we start playing, it is obvious that I am hopelessly outclassed but with some luck of several lucky leaves and a couple 9's on the break---I win the hill/hill set. I know that if we play another set its over for me and I will have won nothing. I am not as good as the other player, I have no confidence, and $100 is important to me. I also know that there is no cure for stupid. What would you do???