Well, he comes up on the fourth anniversary of his appointment next month.  Gerhard Cardinal Müller indicated in an interview that the Holy Father was not reappointing any of the dicastary prefects to more than one term.  If that's the case the Holy Father will simply let Robert Cardinal Sarah's appointment expire and replace him with little fanfare.  At that point, the Pope would just need to find something productive for Cardinal Sarah to do in the last year and a half before he is required to resign due to age.