Realistically, Roland Garros has over the years proved to be the hardest slam to dominate, since stamina, speed and warrior mentality is such a big part of winning this GS. For that reason alone, I think that Rafa will find it very hard to win more than 6 Roland Garros titles, which is pretty damn incredible in itself. But in a few years time, someone younger, faster and hungrier is bound to come along. For a long time now, noone except for Rafa has been able to dominate te clay season for more than a couple of years.
My guess is five or six. At some point around 24-25 years old his legs wont be the same. I hope by that time he has managed to get some wins in other slams, mostly Wimby, so he isn't remembered as a claycourter only.