When a person is sentenced to a certain number of years in prison, this number is not set in stone. There are many different ways that inmates can secure an early release from prison. Some people may have their sentence reduced as a part of reducing jail overcrowding or because they become old and difficult to care for inside a prison environment.
Even after serving part of a conviction an inmate may seek a federal sentence reduction by taking his case to a federal appeals court. A defendant can bring up new evidence that can prove their innocence. After a certain point, many people become eligible for parole, which will also reduce the sentence. Once you are already serving a prison sentence there is still legal help available to reduce the time you’ll serve.