That's just way too long, really. Even if I love the game, by that time I've moved on so thoroughly that the odds of going back to it for DLC is pretty low. It'd have to be Half-Life episode-esque, where the DLC was practically a sequel, for it to be appealing.
Really, it sounds like they will make the DLC a digital only expansion pack. People buy expansion packs for other games. I'm sure if it has plenty of content people will be interested in it.