So, about 10 years ago, I experimented a few times with myself in the summer. I just went on the ground in a remote place and left it there to see what would happen over time. (Not any TP, of course.) It was always completely gone within a week. Bugs and bacteria, I assume. I'm sure the experiment would turn out differently in different climates, seasons, and certainly with higher use. I wouldn't invite my neighborhood for such "visits" on my land.
I've also had two deer corpses on my land. One I buried with my backhoe. The other, before I had a backhoe, was completely assimilated within a couple of weeks. Even the bones eventually disappeared somehow.