The "official" policy is: unlimited refills for the length of stay in which you purchased the mug at the resort you purchased it at...
many people use the mugs on return trips and many people also use whatever cups they happen to have....I have never seen anyone catch any static at all no matter what they've decided to fill, so , go figure...
To my knowledge, the mugs are NOT refillable in the parks, only at the resort in which you purchased it..