Pollard has to appeal to the US President for clemency. He is the only person who can release him.
Usually such clemencies are given by the President After the election in November for his successor and January when the new president takes office.
Clinton gave out a whole bunch of such clemencies like Mark Rich and someone from Kiryat Joel.
Bush only gave one to some border agents who killed a drug dealer (Pollards name DID come up during that time)