At what point would you physically defend yourself?

Currently here in Scotland there are discussions and links on various forums and Facebook to a video showing a teenage girl bullying and then punching a boy several times in the face. It was filmed by one of her friends and posted online, but now it has kicked back on them with the police involved etc.

Looking through comments about it, the one thing that comes across is that the majority of people feel that the boy did amazingly well with all of this provocation not to hit her with an arsewinder punch that would knock her into the middle of March and this links into the idea that as a guy you never hit a female.

This is something that I agree with and never would, but then I would hate to hit anybody, but it does raise the question as to when would you physically defend yourself? We are told you should never hit certain types of people, but where is that line at which point everything changes?