I think he can keep up for another year. But playing many tournaments got him into this form, whereas playing many tournaments in the end also got him back out of form a little bit. A lot also depends on how many fast surfaces there are around. Without the roof, basically all four Slams were a bit too slow for him. Roger needs a bit of pace to work with to be able to beat the top guys.
Anyway, I think next year will be fine. Maybe even win a Slam. After that it'll get tougher and tougher. I think with his serve and forehand on a good day, as well as improved volleying skills, he'll still be a threat, but not as much as he is now. I see an Agassi like career if he wants to play like that. Some early exits, but also 1 or two Slam semis/finals or even a win very late in his career. The guy is so extremely talented. He'll become streakier and streakier, but when he gets redhot there will always be few players that can keep up.
I'll be optimistic: He'll win a Slam in 2013, make 1 slam final in 2014. And then in 2015 and 16 he'll win a final Slam to finish at 19, as well as another olympic appearance (and hopefully a medal in 1 of three disciplines) in Rio