Question: "Time off for good behavior" is a phrase that gets thrown around a lot, but what, exactly, does it mean? And how much do you get off in the federal system?
Answer: Many prisoners can get time off—that is, a reduction in sentence—by behaving well. In the federal system, prisoners who, in the judgment of the Bureau of Prisons, have exhibited "exemplary compliance with institutional disciplinary regulations" can get up to 54 days per year off their sentences. Prisoners are eligible if they are serving a prison term of more than one year; those serving life sentences are ineligible. Earning or making progress toward a high-school diploma or the equivalent is a factor that weighs in favor of time off. (18 U.S.C. § 3624(b).)
The Sentencing Project cites a study by Sabol and McGready finding that, all factors considered, someone entering federal prison in 1997 "could expect to serve 87% of the sentence before release."