It's very hard to predict until we see how he's playing when he returns. We don't know how well Federer and Nadal will be playing next year, either, not to mention Murray and Wawrinka. And at some point, younger players will start winning slams.
This life, which had been the tomb of his virtue and of his honour, is but a walking shadow; a poor player, that struts and frets his hour upon the stage, and then is heard no more: it is a tale told by an idiot, full of sound and fury, signifying nothing.
I believe Djokovic will retire sometime in 2018, probably in the middle after he bombs out of tournaments in the early rounds and he realizes he isn't talented enough unlike Federer or Nadal to compete in his 30's. Maybe he hangs around enough to play the US Open but he'll probably bomb out in the first couple of rounds (QF at absolute best).