How many times can you pillage the same province in Blood Rage?
As much as they have rage to do so.
From the official FAQ on CMON's site:

Q. Does Tyr’s Challenge (pay 2 rage to repillage) allow a player to gain the pillage token reward more than once in the same age?
A. Yes, with Tyr’s Challenge you can pillage an already pillaged province, as long as you pay 2 Rage every time.