Since 1955, if not earlier. My "Webster's New International Dictonary of
the English Language," 2nd Edition, unabridged, 1955, gives "Ur" as a
prefix, "signifiying primitive, primeval, original, as in Ur-Hamlet."
(From the German, of course.)
My-brother-the-word-maven confirms my sense that the usage has become
increasingly common "in English," but adds that it often takes a while til
dictionaries include these things. If you have access to a recent Random
House edition, try that, as he says they're pretty good at being uptodate.
Judy