This could explain why you need to release an addon at least once a month to "extend" the expiration.
I personally made it a habit to check the Carbonite website for any updates so I have little to no problem about "the expired warning". And if I did start the game and the expired warning showed up, I would just log out and update my version. Usually doesn't take more than 2 minutes. What does take long is the starting the game again. :) Can't win them all. :D
I do however play on a different server than my "Authenticating Character". In my authenticating character's server, I only have Carbonite as the active addon so validation is quicker. In the main server I play, I have all of the addons that I want enabled (including Carbonite).
I think the 6 month subscription allows you to DOWNLOAD the versions for 6 months. The addon itself might have a hardcoded expiration date (which Faatal did say they might increase). But, I think as long as they update (even just to extend the expiration of the addon) once a month all will be good.
I do agree with Faatal that users can and probably will use the addon AFTER their subscription as expired (ie. not able to download the new versions).