Well, from a purely personal point of view, if I hear the word 'stop' said in an even remotely serious tone, I stop. There's just no fucking around with that.
From a legal perspective, though, the issue gets murky. First, it's automatically a he-said / she-said thing, and it's even more difficult, because at the point this would be brought in, both parties would have had to agree that the sex that point was consentual. So there's likely no witnesses to the actual act itself, as opposed to people seeing a guy pulling a girl along or something prior to a rape. As well, how long do you give the guy, as most guys in that situation (assuming she really said it, and he had no intention of being a rapist) would either hear and stop, or possibly not hear it (if she said it quietly enough), or misheard it (as in "don't stop"). I know the latter is easily done. Not to sound creepy, but I overheard my neighbor having sex in sophmore year - got freaked out, because I thought I heard her say 'stop', and so I rushed to the adjoining wall, to see if I needed to call the cops or something, and heard her saying... well, I won't get into that, but she definitely wasn't asking him to stop.
EDIT: To the above, there would definitely be a buffer period of some amount of time, but I think that (unless the guy was a really damn quick finisher) there would be ample time between the withdrawl of consent and the guy finishing up that he would realize her intent and be legally (and one would hope, morally/ethically) bound to stop.
Also, the claim would likely have not physical evidence, either, as there would definitely be signs of intercourse, but, again, both parties had agreed that the sex started as consentual.
Honestly, I have no idea how they'll rule on this. I don't know how they could. Obviously something must be done, because any guy who keeps going knowing that the woman doesn't want him to is most definitely a rapist, but how it's going to be implemented is beyond me.
~kov.