how old does a corpse have to be ...

to become available to archaeologists?

like, King John must be getting up there (d. 1216).

http://en.wikipedia.org/wiki/File:King_John%27s_tomb.jpg

http://www.nydailynews.com/news/world/2010/07/05/2010-07-05_bobby_fischers_body_exhumed_for_paternity_test_filipina_woman_claims_he_fathered.html