I've met this one, well pressed my face against his glass for a good hour or two. He's from 300 BC or so. He still has his eyelashes. Even this picture doesn't do the preservation justice. It looks like he fell asleep, save the rope on his neck.
I'm sure it was accidental at first, but eventually they found out bogs were great at preserving things. There are plenty of records of people putting food in bogs to preserve them.
Having thought a little more, the body would likely stay pickled in perpetuity as long as conditions are stable. I do not know what these thresholds are for bodies, though.