Bit of a roundabout answer here, but be patient with me:
TWD's zombies are clealry Romero-type.
Max Brooks, having studied Romero-type zombies extensively, wrote The Zombie Survival Guide and World War Z.
In these books, Brooks actually addresses these issues. It boils down to: Zombies have a limited lifespan, generally. However, it's longer than it would take a normal body to decompose. Part of this is because they generally aren't in anaerobic environments like regular bodies. Part of it seems to have something to do with changes in the biochemistry which causes them to partially desiccate, which acts to preserve the bodies. They have an average lifespan of up to ~ 10 years in normal conditions. However, zombies in abnormal conditions continue to persist for reasons unknown. As an example: zombies who roam the ocean floor continue to survive more or less entirely intact, and nobody knows why.
Now, if we're talking Rage-type zombies, this is a different story entirely. With their ridiculously heightened metabolism, once most people had been turned/eaten, they'd die off in a matter of weeks, maybe a couple months.