This was a continued topic of debate/confusion amongst my family. Whether or not the refillable mugs are for "life of the cup vs. length of stay." We could not find anything specifying one way or another. As
@CastAStone stated in post #2, "They are neither advertised as length of stay nor for forever. The signs simply say they are limited to Aulani mugs."
Here's what I'll say, when we bought our mugs, the CM at the register literally told us, "Whenever you come back to Aulani, bring back your mugs and you don't have to buy another one." So I'm going to personally run with that until instructed otherwise.