How much of a federal sentence must be completed before parole?

Generally, before any parole may be granted, the minimum sentence, less any administrative good time, must be completed. I am not familiar with any Federal Parole system. As I understand it, Federal time is calculated on a one for one basis; that is, for every day of time served with no administrative disciplines, the inmate earns one day off his sentence. Therefore, for a ten year sentence, the inmate may only have to serve five years.