I'm actually surprised Nadal has been able to play as long as he has thus far. If he schedules intelligently, I think he can be at a slam winning level another 3-4 years. No more than 5. Then again, he may retire tomorrow, I dunno.
But he's got a few more years, I think.
We've been talking about Nadal burning out early since 2005. A lot of people would use Hewitt as an example when talking about Nadal back in 05/06. I lost track of the number of times his fans feared for his health.
I say this not as a Nadal hater. Specially after I read his bio. But Nadal has two amazing abilites:
1- Play through pain
2- Exagerate his health issues
One might overshadow the other depending on if you are a fan, a hater or neutral.
Given everything in consideration it is safe to say that Nadal didn't burn out as soon as people predicted. It is not likely that he will play beyond 30 yo. It is unlikely that he will stop at 11 GS titles.
Unlike Federer, Nadal is yet to go through an entire year without winning a GS title. I suppose we can start talking about a decline when it happens. And this will be moment when we can seriously start thinking where the finish line is for him.