Those are known as expansions and happen every 2 years and will for a very long time.

They say expansions will be developed for at least 5 or 10 years, but the population of the game and the fact it has grown at every expansion means it will be strong enough to get expansions for at least 10 years, because even if the population declines, the amount of people invested in the game with mounts, achievements and memories will make that decline slow.

Even the other MMO FF11 has been getting some major updates recently after they were stopped for years and that's after 19 years.