When it does happen, how long will it go on? I mean the body will start to decomp faster & faster as the weeks go on. So the question is can you just wait it out?
Have to agree with Brigitta... what it's really going to depend on is how long it takes for people to start taking shelter and stop taking stupid risks. Even then, there will always be accidents, stragglers and such. A year's a good guess... but that's probably optimistic.
And, we're of course assuming that whatever makes zombies, be it virus or whatever, doesn't mutate into something that can be transmitted another way (say, airborne).