Depends on many factors
Inmates can be released before their release date but there are many determining factors.
A man I know personally was sentences to 60 years for manufacturing and was released after serving 6 1/2 years. He was a model prisoner (worked and joined classes and programs...kept his nose clean and stayed out of trouble).
I did some research on this and again, depending on the conviction (for what reason) the state requires that they serve at least 1/3 of the sentence before becoming eligible for parole; however 1/3 of 60 years is not 6 1/2, so who knows.