Thats a leading question. 3 days down time, theoretically you could do a straight c check, working 3 shifts on it with little or no additional work...but that never happens. depends how old the aircraft is, how many snags you find, a week would be more accurate I would say.
KLM are probably leaders in that game, mostly contract engineers doing the work, but it would be quick and a pretty good aircraft when it came out.
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Old age and treachery will always triumph over youth and enthusiasm.