I don’t know what happened. I got engulfed in this answer. The more I dug, the more I realized that a Zombie lifespan was not that long.
It all started with watching the First season of the Walking Dead. I started thinking to myself what I would do if the Zombie Apocolypse happened. I did some math – to kill the US population in zombies alone would take hundreds (if not thousands) of years.
But then I started thinking – what is a lifespan of a Zombie? Especially one that is broken and crawling on the ground. At the most, it would be one generation, because they couldn’t be able to reproduce.
I started doing some reading and realized that unless some supernatural force was at play, the body will break down quick. The best condition for zombie survival would be up in a frozen wasteland, otherwise, their time is really limited on this planet.
If you were to hole up in a building made up of steel and concrete, with impenetrable or no windows, no access to the roof from the ground, and enough food and water for over a year, you could probably survive the apocolypse.
The conditions would have to be prime – Zombies would decay slower in the winter than the summer. A hot and humid area like Florida or Louisiana would be perfect. You would also be protected from the south – if a zombie tried to walk across water – that could literally tear them apart much faster.
To give you a better idea, I have concocted this Infographic – a timeline from bite to full death with a CSI twist.
The end result – within 4 months, any zombie out in the elements would be confined, for their body would be too frail to move. Within 2 years, the zombie population would be mostly immobile. Especially enough for you to go out there with a flame thrower and torch the remains.
Thanks to Joseph Lee for some help on the Decomposition timeline