In the US, the law varies depending on jurisdiction. Federal law does not speak to self-defense rights except in the larger sense of civil liberties.
However, generally speaking, a person has the right to defend themselves if they are
reasonably in fear for their safety.
Period.
There is no
'21 foot law' or
'if he raises his hand to you' rule, nothing like.
People who do not know what they are talking about make that crap up.
The right to self defense is generally enumerated in state or municipal law. And the most typical description is that it may be used according to the 'reasonable man' standard.
http://en.wikipedia.org/wiki/Self-defense_(theory)
Please click on the above link and refer to the section labeled
"Legal status of self-defense."
Please note also that it is not a matter of what YOU consider a 'reasonable fear', the standard is what a mythical 'reasonable and prudent man' would be in fear from.
http://en.wikipedia.org/wiki/Reasonable_person
Nothing is cut and dried - cases are evaluated on their merits, and if you defend yourself with force, you may find that you have to defend your actions to a police officer, a DA, or in court.
Getting back to the original question - if you have the right to defend yourself in a given situation, you are under no obligation to wait until the other person actually hits you.
Bear in mind that the right to self defense may not be absolute. Some states have a 'Castle Doctrine' or so-called 'Stand your ground' law that does not include a duty to retreat - some do not, and you are actually required to retreat to the extent that you are able before you can defend yourself legally.
Also, having the right to defend yourself generally means you are only allowed to use
'that force which is necessary' to end the danger. Roughly speaking, that means you can't beat the guy with a baseball bat if you actually tower over him and can easily subdue him with empty hands.
Self defense also means you have to stop when the danger stops.
Self defense also means you can't 'revenge' for self defense. If the danger is over, your right to defend yourself is over.
And of course, remember that martial artists may be held to a higher standard regarding minimum amount of force used - just so you know.
OK, summary:
You generally have the right to defend yourself against dangers that would make a reasonable and prudent man afraid for his safety. If that is true, you do not have to wait until actually attacked.
The right to self defense attaches to the danger, not the actions taken against you.
I am not a lawyer, this is not legal advice.