There is always a huge debate this as some people bring back old mugs from previous stays and say “when I bought this mug there was nothing that said it was only good on this stay”, and then there are those who say that is wrong. There is usually a debate this on any Disney message board.
So you buy a “new resort mug” at the food court it will have an RFID chip in it. When you go to use the mug it will read or scan your mug’s RFID chip and you will get the drink. I am guessing if the mug is not valid then you will not get your soda or drink and it will be valid for the dates of your stay at your resort. So no more bring back old mugs and filling them up! Also there will be a time limit so that you have to wait so much time in between refills.