The true answer is never, as you shouldn't be considering it as rejection.
jajaja, nice way to put it. Maybe a more appropriate question for guys who are good at this already would be:
"How sure are you about being able to go out and bring a good looking girl back to your place for sex?" (50% sure, 80% sure, etc.)
PS: By good-looking I mean a girl that you find attractive to want to have sex with (HB10, HB9, whatever you think is hot enough to make your lover gladly. It's just a way to define the scale of hotness in two words