"Well, this makes a lot of assumptions. The traditional zombie archetype is a still-rotting corpse that eats without requiring food, water, or oxygen and cannot be killed through any means other than brain destruction and/or decapitation. But let's play with this archetype.

Assuming that the corpse rots at a normal rate for a human body (with the assistance of insects), after 20 to 50 days the zombie will be mummified. This means that after only 2 or 3 weeks, the brain will have rotted to the point where the zombie begins to lose motor functions and finally dies on its own. There's a lot of variables that can affect this, but after about a month it's safe to say that a zombie of this type with nothing but natural decomposition and insects to wear it down will be dead.

Now, this is still taking a lot of these assumptions into account. Scientifically, the most likely zombie is a 28 Days Later-style rager that is still alive and human, but driven into a rabid rage against all living things. These are actually far easier to deal with, as they're vulnerable to mortal wounds (both man-made and accidental), fire, drowning, starvation/dehydration, and potentially infected wounds. Not to mention that with starvation/dehydration comes the weakness that they simply cannot overcome with adrenaline. Within a few days, one of these zombies will be nearly dead from lack of water. Even if it requires no water but still requires food, after a week or two it'll be low on energy and sickly. A rager that makes it to 3 weeks is elderly."

"Shamblers are in a sense harder to kill; if they can only be killed through brain damage and decapitation like the standard archetype, that's a smaller target to hit. A rager can be taken out through the same means that eliminate humans (none of the pain-related effects will probably work, though) and they have a shorter individual lifespan. They also suffer the same fatigue effects as humans, while shamblers are essentially tireless. If you have a suitable fortification, you really only need to last a few days before the first horde dies of thirst."

"Due to their lack of ability to heal wounds, they'd most likely die within a week of being zombified. Ragers would die quicker than shamblers, as they would rack up more day to day cuts and scrapes, cumulating quicker to amputations and ultimately death"

",Zombies, their dead right?. Decomposition starts at death. If they don't eat us wonder how long will they live? how can they live if their dead. "BODY ROT"S". john"

"Honestly, I believe a worst case scenario. While my wife, and many more, may disagree heavily with me on this, I don't believe zombies are rationally possible. Instead, I believe they will be brought back by supernatural forces, like black magic. I also believe that there's a good chance running will be enabled for them, and that the magic might stop all decomposition for them and maybe even fatigue.

But if that is what brings back the dead, I think zeds will be the least of our problems."

"In reality, only some forms of supernatural force can animate a dead body."