It really depends on size of the team and equipment. Three guys with a hand saw, the trees will probably grow back as fast as they clear them. Three hundred guys each running an industrial feller, that's done by lunch and they've got time for some kebabs (maybe an exaggeration, but still).

If they snuck in a large enough team and clearcut, it's very plausible they could do that in a week or two without someone visiting and noticing.