What Monique is talking about -
When bodies are buried in boggy soils, the tannins in the soil water tan the skin, and after a while the skin is preserved and the bone gone. This can happen fairly quickly. In this case the acidity of the soil actually dissoves bone mineral.
When bodies are buried in very dry regions, and exposed to salts like natron, most tissues, including bone, can persist for thousands of years. Egyptian mummies come to mind. There are skeletal remains of early homo sapiens from the Northern Sahara desert region - ~90000 years or more that apparently have not been remineralized (fossilized).
So. Until you can be more specific, this is about as far as we can go with any details. IF you look up how bone becomes fossil it might help.