If my fiance gets sentenced to 33 months in the feds how long would he actually have to do?

The federal sentences come with 15% goodtime credit going in.  If your fiancé stays out of trouble he will serve 28 months. If he has an alcohol or drug problem he might qualify for RDAP, which if taken would cut 9-12 months off the 33 months. Ask the Inmate about RDAP.