Are you looking for a legal answer? Technically, the rule for rape-by-force can be explained as requiring you to stop as soon as there is no longer consent. If some person was wacky enough to withdraw consent in the midst of penetration, then I can't imagine you'd be guilty if you acted quickly to stop intercourse. But no, you don't get to say "Just a few more thrusts, please." In short, the test is whether you are trying to stop.