Hi BWIA 772, Buzz here. At UAL, C checks are more-or-less a yearly inspection that takes a couple days to work through.
The D check is a heavy overhaul that can take weeks. We used to break down the D check into 1/4 of it's original size inspection,and cycle the airplanes through 4 times more frequently. If you let the D check interval run to the very end, we've found some severe corrosion on customer's airplanes. Then the airplane is parked for 5 or 6 weeks while belly / floor beam repairs are made.
Pardon me, it's been 15 years since i worked overhaul. I much prefer the life of a line mechanic working midnight shift, troubleshooting anything that comes along.
Buzz Fuselsausage: Line Mechanic by night, DC-3 Crew Chief by choice.