Concorde @ 18 hours * 6 hour round trip = 108 hours of hangar time.
4 1/2 work days of maintenance per flight? Really?
It's in man-hours. Presumably British Air had more than one guy working on the Concorde. (Not sure about Air France though). If you had 15 guys in a crew, 108 MMH would be done in a shift. My guess is that afterburning engines for a plane that is supersonic are very maintenance intensive, and that is where the bulk of the maintenance man-hours stem.