The one charge he was convicted on carries a amndatory minimum of 20 years in prison without parole, and a maximum of life.
Do you have a source for that?
For a first conviction I find 0 to 20 or 25 years.
http://codes.lp.findlaw.com/uscode/18/I/40/844
http://www.law.cornell.edu/uscode/18/usc_sec_18_00002332-—b000-.html