If FFXI is any indication, I would say that actual support for the game won't drop until they make a new "successful" MMO Final Fantasy game and had it run successfully for a few years, so there's definitely over a decade left. Granted, that's not to say when you yourself will be done with the game since people experience diminishing returns differently, and while the experience doesn't have to dwindle for you, I'm certain that some may grow tired with it's setting or become upset if an expansion comes along and drops a ton of unwanted changes.
