Perhaps a better question would be "How many math-months does it take to prove it?"* It's been three years since the Perelman program was announced, about 30 since Thurston's geometrization conjecture, and 102 since Poincaré first declared that the 3-sphere was unique among 3-manifolds for having trivial fundamental group; there were, of course, many more stops along the way. The answers to those questions are probably much larger than I would care to guess.
So, who's working on P vs NP?
*While the man-month is said to be mythical, I suspect the "mathematician-month" to be a realistic unit of measurement.