During the Reagan boom years, a developer bought land in a town that had been a big tannery area until the early 1900s.
80 years later, backhoes were snagging still rotting carcasses of animals.
Of course, the decay could have been slowed from all the leftover chromium and arsenic sites.
Back in the early days they used mercury in tanning which almost entirely stops decompostion.
I think a burial of a large number of cows would take decades to be turned into only skeletons...
But it would be a great place to plant a garden or tree's...