Contact the place at which the person is serving their time or ask the inmate. If it is a felony resulting in prison time, the only thing they can tell you is an estimation (and that all 3g or aggravated offense must serve at least 1/2 before becoming eligible for parole.) If it is a misdemeanor, then they should be able to estimate an out date. (I say estimate because inmates occasionally lose their good time credits or get more than some other inmate depending on classification.)
The answer will depend on what type of charge the inmate was convicted of. If convicted of a misdemeanor, the inmate will serve time in a County Jail. Most Counties allow inmates to receive 2 days credit for every day that they serve in custody. That credit can be increased if the inmate is considered a trustee. If convicted of a felony, the type of offense will determine when that inmate is eligible for release on parole. For example, if convicted of a 3g offense, the inmate will not be eligible for release until half of the sentence is served.