|MadSci Network: Anatomy|
An unembalmed bodies buried six-feet deep in ordinary soil without a coffin takes ten to twelve years to decompose down to the skeleton. It takes about 300 years for the bones to fully decompose under those circumstances. Bodies left on the surface in temperate climates could decompose within four to six weeks. The higher rate is due to the action of insects and animals eating away at the bodies. Embalming slows down the decay of the body by a factor of two to ten, but it doesn't alter the rate of bone decay. Check out the book "DEATH TO DUST" by Kenneth Iverson, published by Galen press.
Try the links in the MadSci Library for more information on Anatomy.