Novak is now 29. He will turn 30 on May 22, 2017. See chart below for how people have performed after going over the hill. Fed and Pete won one Slam, Rafa not even that. The general playing style of Novak has more similarity to that of Rafa, than to that of Pete or Fed. Let us give even 2 slams to Novak after he goes over the hill, due to lack of competition, his flexibility etc.
So, I would estimate Novak's total to be bounded above by
12 + (However many of the next 4 ending in RG17 that he wins) + 2
My (totally unscientific) estimate for the middle part in the above equation is 2.
Here is the distribution of slams won at different ages (going back to the beginning of open era and ending in AO 2014 as the graph is little outdated). Since the beginning of 2000, only 5 slams have been won by players over 30 ( 2 by Agassi, 1 by Sampras, 1 by Federer and 1 by Stan). So, recent history is even worse than the total history in this regard despite the fact that GS draw these days have lot more over 30 players (this RG had more than 50 over 30 players).