Dario D. said:
a young man beats up a girl about his age, breaking several bones, after finding out that she assaulted his sister. After doing something like that, can you expect the police to be on your doorstep within minutes? And, if so, what kind of jail-time are you looking at? Is it something that would go to court?
As Rabe pointed out, the legal definition of "assault" is different than the normal definition of "assault." What you're describing--actually physically injuring someone--is "battery," in legal terms (as in battering a person, a battered wife etc.). Legally speaking, assault is scaring someone by making them think you're about to hurt them; battery is actually hurting them. So, generally people who commit this crime are charged with assault and battery, because they do both (merely raising your fist like you're about to hit someone is assault, if you're doing it in a way that would make a reasonable person think you really were about to hit them).
So it sounds like what you've got there is assault and battery. Given how badly hurt the girl was, maybe it's aggravated assault and battery, I don't know--the exact definitions depend on state law. It could even be attempted homicide, if she's hurt badly enough or he purposely inflicted injuries on vulnerable places (e.g. kicking her in the head...). Do you have a law school anywhere near you? Most law school libraries are at least in part open to the public (i.e. the public can come do legal research, though they can't check out books), so you could go ask the librarian (who is not a lawyer but is knowledgable in law) for help with this question.
He could indeed go to prison for a multi-year sentence; it's hard for me to imagine any place that wouldn't treat that kind of crime as a felony. (A "felony" is a serious crime, defined in most states as any crime that COULD BE punished with more than one year in jail. People charged with felonies are sometimes sentenced to less than one year in jail, like if there are mitigating factors, but the fact that the crime COULD be punished with a year or more means it's a felony. So keep that in mind--after a conviction on this charge, this guy is a convicted felon: he can't vote, can't own a gun, etc.) He could also get a heavier sentence if he's got a history of criminality; judges take that into account in sentencing.
Also keep in mind that most run of the mill, non-famous cases end in plea deals. That means the prosecutor bargains with the defense attorney: "I'm charging him with [a whole bunch of different crimes, the worst stuff possible under the circumstances], so your client is looking at 12 to 15 years. But I'm willing to drop it to [lesser crimes], which would give him only 3 to 5, if he pleads guilty." In the case you've described, the prosecutor might threaten to charge the guy with attempted murder but then offer to make it assault and battery if the guy pleads guilty.