There are several variables involved. If this is state where there is a parole board, it would depend on what kind of crime it was, how many times they have been to prison before and how many disciplinary actions the inmate had received.
If this is a state where there isn't a parole board, at times they have rules that you would be released if you have met your "minimum time". It's hard to tell without knowing what state you are in.
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