i think what you want to know about is "good conduct" credits which help reduce a sentence by a certain amount of days each month
the US Code 18, section 3624(b) provides that a prisoner who is serving a term of imprisonment of more than 1 year other than a term of imprisonment for the duration of the prisoner's life, may receive credit toward the service of the prisoner's sentence, beyond the time served, of up to 54 days at the end of each year of the prisoner's term of imprisonment, beginning at the end of the first year of the term, subject to
determination by the Bureau of Prisons that, during that year, the prisoner has displayed
exemplary compliance with institutional disciplinary regulations.
however, a maximum of 47 days of good time for each year of the sentence imposed is granted - and under the Prison Litigation Reform Act, prisoners convicted after April 26, 1996 and how much good time they may receive depends on whether they have earned or are pursuing a GED or high school diploma and if the prisoner has earned or has made satisfactory progress toward earning a GED or high school diploma, he can get a maximum of 54 days good time credit on each year served.
and, if the prisoner has not earned or has not made satisfactory progress toward earning a GED or high school diploma, he can get a maximum of 42 days good time credit on each year served.