Establishing a time frame for the zombie lifespan is a central area of focus for the Zombie Research Society (see: Fresh Stage Decay & The Zombie Lifespan). With that in mind, ZRS Researcher, Rick Lowe, suggests that the undead may increase their longevity by literally eating themselves from the inside out.
Lowe argues that many of the body’s internal organs – vital to the survival of a living human – are not needed for a zombie to function. Because of this, the newly reanimated corpse has a portable store of food built right into its framework.
“Humans can go weeks without food. But as they eventually reach the later stages of starvation, the body will literally eat itself. In fact, digestive-related organ failure is what usually kills a starved person. Zombies likely have no need for their organs, so they are free to digest the heart, lungs, liver, and intestines without suffering any ill effects, thereby substantially increasing their lifespan.”
Regardless of whether or not zombies actually swallow the human flesh they bite into (see: Do Zombies Really Eat People?), if Lowe’s digestion theory is correct, then zombies can avoid going “hungry” for some time, making thei lifespan a matter of months at very least, and potentiall a great deal longer.