How much time will you serve in federal prision if you were sentenced to 5 years?

Last I heard, federal inmates got 54 days off a year for good behavior. They have no parole. It's called pre-release and most do half way house time at the end of their sentence. So a rough guess would be to knock 9 months off the 5 years. But if they got the last 6 months at half way house, they'd be in prison for like 3yrs and 9 months in the pen and the last 6 in HW House. This is just a rough guess.