Why? I don't think that there is a rehabilitative aspect, necessarily. When you have a young person getting one, what are they doing? They are going to go into a jail or a facility. They're going to associate potentially with people who are hardened criminals and have committed criminal acts. They're going to associate with a criminal element; you're not addressing the underlying social root of the problem. What you're doing is teaching a person a certain way of life. The longer you expose them to that way of life, the worse it's going to be for us as a society when they get out.
Longer periods of incarceration are going to be problematic for exactly that reason. What you're doing is teaching criminality, potentially.