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It depends very much on the conditions where the body was found. Humidity, insect/rodent activity, and a number of other factors can greatly alter the time needed for remains to skeletonize. It can range from a number of months to a few years (more if anything was done to preserve the body like embalming).
"Although the remains are skeletonized, it is not clear how many years they have been there," a University of California, Berkeley spokesperson said in a statement.

I'd wager pretty long. I'd google it but then I'm on some list.