How long could a zombie apocalypse actually last?

[ChiggyvonRichthofen]

As others have pointed out, it depends on the nature of the outbreak, the nature of the zombies, how many people are infected, the timing involved, and so on.

In most scenarios, they really have to make the various militaries around the world ridiculously incompetent for the zombies to actually be a threat. I'd agree that a lot of writers don't really seem to appreciate just how quickly huge groups of zombies could be eliminated by even a small number of soldiers. Of course, they usually know better than to show too many soldiers, but it's pretty much left unexplained why the military isn't making quick work of the zombies and coordinating the evacuation and safekeeping of civilians and survivors.

Just as egregious as the incompetence of the military is the apparent competence of the zombies in dealing with the military off-screen. Brainless dead beings who may or may not be able to run seem to be able to consistently find weak spots in defenses, rush fortifications and work together, appear in overwhelming numbers at convenient places, and so on. I don't see how it could not be a turkey shoot. I'm not a huge zombie fan, in part because the story is just too contrived in too many different ways.

[yamiangie]

I think in the TV Show the Walkers appear to be actually decaying so while it's slower than actual human decomposition at some point the brain will liquidity. Apparently in real bodies it's like 20 days for that part of decomposition to finish, which is likely why zombies don't decompose like normal dead bodies. At some point even if the brain can't fall out the zombie should rip it's decaying mussels apart trying to move.

[Independent George]

I think the fundamental problem is this: zombie apocalypse stories always center around a rag-tag group of survivors. Zombies weak enough for the group to believably survive are going to get wiped out effortlessly by professional military. Any zombies strong enough to defeat a modern military is going to effortlessly wipe out a small group of survivors. It's exceedingly hard to balance between the two in a dramatically satisfying way.

28 Days Later works for me because it was extremely easy to believe the military would be overwhelmed, and the survivors are appropriately terrified of them. The result were zombies so strong that the only viable military solution was "survive long enough that the zombies starve in the winter", rather risk any offensive operations they are likely to lose.

Because World War Z is about the global response to the zombie apocalypse, much of it has to be about why the conventional military is ineffective. In order to preserve drama, it needs to find a way to make the zombies powerful enough to defeat a modern army, but not so powerful that it can't be beaten later in the third act. So it tries to have it both ways by making the early failures a part of a hard-earned learning curve... except, because the real-world military has already gone through that learning curve, they end up looking like morons. It's the equivalent of Superman forgetting he can fly.

[FaxModem1]

In most cases, the military grabs the Idiot Ball with both hands. Fear the Walking Dead is a huge example of how NOT to deal with a crisis. It also casually villainizes the military every chance it got, even though it was utterly unrealistic.

World War Z's excuse, and one I'm willing to buy, is that the US military was already severely depleted from a previous war, military spending was cut, and politics played a big part in deployment in the same way that it was during Afghanistan and the Iraq War. It's why one key chapter with the former Vice President is about the military recommendation, and what actually got approved. Part A was 'Alpha Squads', small teams going into hotspots and taking out the majority of the zombie population. Part B involved a lot of deployment, military troops being stationed, etc. Congress and the President said no, because long deployments are unpopular, it would affect the budget, etc.

So, military was willing to get stuff done, but hamstrung by Congress over and over again on what they could deploy and use. You kick the can down the road on an issue like zombies, and eventually it bites you back(almost literally).

It's why those who critique the Battle of Yonkers are right, if the battle happened tomorrow. if it's in a situation where the military's hands via ROE are being tied, their budget slashed, and they're already recovering from another war, the answer is a bit different.