It depends on the conditions where the body is. For example, a frozen body will decompose much more slowly than one in a hot and humid environment. But basically, once life stops, the bacteria w/n our bodies starts to consume the flesh. If unattended, bugs come to eat the corpse, as will any other wildlife. There are lots of books out there that describe the process of decomposition; off the top of my head, Mary Roach's "Stiff" is a decent place to start. She's a very good science writer.
