When I was on exchange at Boscombe Down, and we were doing a lot of Doppler testing, compass swings were done to a gnat's whisker of precision. This was necessary to know what was affecting the accuracy of the nav system. THree hour compass swings were not uncommon in the test Sea King - rotors running, all systems up.
A couple of years later I'm at Pax River and watch the USN do compass swings on their H-3s with blades folded and no engines on, and they wondered why their Doppler nav systems never were very accurate....