How long should it take to get over a rejection?

Over a rejection? It should be short. Think about it this way. You can't control who you find attractive. There are guys you find attractive and guys you don't. There are going to be guys that find you attractive that you don't find attractive, and there are going to be guys that you find attractive, but they don't feel the same way.

So don't take rejection personal. You don't need a billion guys to like you. You just need one really good one :o) Don't put all your eggs in one basket. If a guy doesn't like you, it's cool, move on.

There is no specific number for getting over someone. Some people get over break ups easily, while others take a long time. It can also depends on how much you put in to the relationship. If you put a lot in you obviously don't want to let it go. If you didn't put in a lot you would care but not as much. (when I say put in to the relationship I mean, sharing emotions, fixing the person up, etc).
It really depends on you and how you feel

Depends on the person, and how bad the rejection was. Your friend needs to realize how much energy she is wasting on him, energy that can be used on other guys who she can be happy with. She just needs to get stronger and move on.