Rehabilitation

When a person commits a crime they need to be rehabilitated. That is, they need to be brought into a condition whereby they are suitable to live in society again. If the only goal of punishment is rehabilitation then no person should be given any fixed term of imprisonment. They should all be sent to prison until they are rehabilitated. The question then becomes, how do we know when a person is rehabilitated? Can we be sure that a person, who is rehabilitated, will not re-offend? If we cannot be sure then is the person rehabilitated and should any person be allowed to leave prison if that guarantee can not be made?