The Walking Dead has been going on since the early 2000s. That’s wayyyyy too long. Theoretically, the zombies would have rotted by now. The decomposition of human flesh is boosted when there is oxygen present. So for an above-ground body, it would take a maximum of 6 months for there to be nothing but skeletons, Yet there are still walkers everywhere. The stages of decomposition are 1. Fresh decay: The organs will begin decomposing around a day after death. 2. Bloating: The bodily fluids will begin to blow up the body. 3. Leaking: The body will start leaking 4.  The body will start to liquify. All four stages occur in around a month above ground.