Although it takes several years for the human body to decompose completely, the process of decay sets in within a couple of hours of death. However, there have been exceptions where bodies of some people have reportedly remained untouched by the natural effects of decomposition even centuries after their deaths.
An Imgur user recently uploaded a few pictures of the bodies "that never decomposed".
Image source: By grooverpedro [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons