Time is not a valid measurement. If you have an AR put away dirty in the safe for 25 years then take it out and fire it, you have an AR that has worked for 25 years without a cleaning.
There was a moderator on the old board that had, IIRC, over 1,400 rounds through one of his without cleaning. Details and name escape me. I'm sure someone will chime in with the details (especially if I'm wrong).[:)]