Parole is NOT a right, but a privilege, and as such the question of whether to grant a prisoner parole or withhold it is up to the discretion of the parole board. Typically the parole eligibility dates are determined by statute, although in some states they can be modified at the discretion of the sentencing judge.
Although the laws vary from state to state, their basic approach is the same. Once a person is convicted of a crime, he is sentenced to a prison term. There are two types of prison terms: indeterminate and determinate. An indeterminate sentence fixes wide-ranging minimum and maximum lengths of time; determinate terms are fixed terms of imprisonment (e.g., 5 years for armed robbery).
If an offender receives a minimum and maximum sentence, once the offender serves the minimum sentence (or serves a specified percentage under some state’s statutes), they may become eligible for a parole consideration hearing. If an offender serves the fixed period of imprisonment imposed by the court, they may be released without further supervision or automatically placed on parole, again depending on the nature and severity of their offense and their potential for re-imprisonment.