I mean, that does sound good in theory, but something has to keep the body moving for it to be a zombie, so either it's not quite dead, or there is some other (often magical) reason at play, possibly also keeping the decomposition at bay. Even disregarding that, though, the 42-day maximum assumes that every zombie would have been infected/died on the first day, so the actual time for it to blow over would be 42 days from the last death. Assuming it only spreads through zombie attacks.