are you serious? I don't know, these things vary wildly depending on all the factors involved. basically you just sort of bring it up in a way that DOESN'T make it look like you've been thinking about it all this time - like you have been - but also makes it seem like you thought it was pretty silly & unlikely - like a normal person.
my theory here is that you're crazy with teenageritis, and we're trying to cover that up here.