On a Delta HMV, which is akin to a 'D' check all flight control surfaces come off. Vertical stab comes off, and many times the Horizontal stab comes off. Vert stab connections get rebushed and new bolts. Interior gets removed including insulation for a complete internal structures inspection.
Most of the cabin door openings get beefed up if not previously done. These areas are prone to cracking. The fwd pressure bulkhead is commonly replaced due to cracks, and many times the aft cargo door gets reskinned. There tends to be heavy corrosion under the rear wet area(Galley&Lavs). Also in the aft bag bin and in the area aft of the bag bin we find a lot of corrosion requiring replacement of parts.
With a experienced crew of of about 160 guys working three shifts it can be done in about 28-30 days.