Well he hasn't necessarily been there too long. Depends on heat and humidity. I've seen a horse that was in the sun for just 2-3 days and the skin sloughed off when we had to remove it. When the rendering truck got there we tried hauling up and all we got in the truck was the skeleton, all the juicy bits were left to me and my shovel. If I had to make an uneducated guess I'd say 4 or 5 days.
Damn......I bet that was one hell of a mess to clean up. I didn't realize that could happen so quickly.