If the following is to be believed, Destiny is one of four games/expansions that will be released every other year. Also, it's a contract with Blizzard-Activision, that lasts for ten years. If it's really "every other year", it'll release similar to the Halo series, 5 games in 10 years (1, 2, 3, ODST, Reach, not counting Wars).
Out of interest, does anyone know if the Marathon story is officially "closed" by Bungie/343? With Halo 4 focusing on the Forerunners, I'd be surprised if they didn't at least make a lot more Jjarro references, let alone the supposed "Marathon" codename, which I somehow doubt is more than a codename.