Last year after US open, Djokovic told the press, that his goal is winning 11 slams, matching Borg, and playing until age of 28.He said that the game in general become too much taxing, and that he doesn't know how long he will last.
At time, he was evidently injured and burned out, so it is reasonable to speak in such a way.This year before Olympics, he changed his story and said he believes he'll play for a long time even in the thirties.I believe, that he's going to win 6 more, at least.
If you just compare him and Agassi, with quite similar playing styles, we know that Agassi played great even in his thirties.He reached 4 slam finals in a row, winning three of them, and he was 29/30 at time.He won 2 more after that, so it's 5 in total from age 29-33.And Agassi had much more troubled career than Novak, injury wise(wrist surgery, back injuries, girls...

)Novak is much better treating his career than Agassi ever did.Even Agassi himself considers Novak as a better player
So, to conclude this, I want to say that it's achievable for him to win 6 more before he retires, he's not going to dominate the game in late twenties, but it's not going to stop him of winning slams from time to time.