That is dependent on a number of factors including hydrology and the mineral content of percolating water, pH, and the nature of the sediments the bones are buried in.
I cant remember the sight,possibly something like Ripleys believe it or not,but I remember seeing examples of bodies being disentered that were only 50 years in the ground,totally fossilized.I think they were in some area where the water level could enter the caskets and was highly mineralized.
Anyway,If tests were done,the minerals would show ages of millions of years but the tombstones proved otherwise.