The math on a straight 25% reduction is straightforward. 25% of 48 months is 12 months. That leaves 36 months, or three years, as the time to be served before release eligibility under that calculation alone.
Louisiana has gone through several sentencing reform efforts in recent years aimed at reducing the incarcerated population and cutting costs, and good time expansions for nonviolent offenders have been part of that picture. If the specific provision you are referring to applies to this case, that 36-month figure is the baseline.
But as the original answer notes, that may not be the floor. Parole eligibility is a separate consideration that can reduce the actual time served even further. If the sentencing judge made the offender eligible for parole consideration, and if the parole board grants it, the person could be released significantly earlier than the 36-month mark and serve the remainder of the sentence in the community under supervision rather than inside a facility.
The variables that determine parole eligibility and outcome are the same ones that apply everywhere. Institutional behavior, completed programming, a solid release plan, and the nature of the original offense all factor into the board's decision.
The most accurate read on this specific situation requires knowing exactly which Louisiana statute applies, whether the judge included parole eligibility in the sentence, and what the offender's current standing is inside. His attorney or the facility's case manager can run the actual calculation based on the specific conviction and the current state of Louisiana's good time laws.