The official word is that your mug is good for the length of stay at the resort in which you purchased your mug.
Yes, they change, yes people bring them back regardless, yes some were told they are good forever, and some were even told at any resort. Yes, Disney seems to not care if you bring them back forever. However, the official word is that your mug is good for the length of stay in which you purchased your mug.
Now, be prepared for a lot of responses..some people think it doesn't matter what you were told and you should follow the rules as they are set up now, some people bring them back forever, since they were told they could, and some just really don't give a darn...figure it's up to Disney to decide when and if they want to enforce it. I'm in the last camp, and I bring mine back. Looking forward to the second thing I do, when I get there, filling my Dixie Landing mug at POR. The first thing will be checking with the food court manager to make sure it's still OK.