Federal inmates must serve their whole sentence, except that they earn a “good time” of 54 days reduction in sentence for every full year served with no disciplinary findings.
So, if a person is sentenced to 45 months, his sentence will be reduced to a little over 43 months after he completed his first year trouble free, to 41 months after his second year, and to 39 months (plus about 18 days) after his third year.
No good time is earned for a partial year, and any disciplinary punishment costs you your whole years good time.