The airplane is practically brand new every C Check and D Check.

Daily Service Check - Basically a comprehensive walk around and computer diagnostic 

Weekly Check - More comprehensive than the daily.  It takes about 3 hours to do and requires an Airworthiness Signoff

A Checks 

Then, every eight to 10 weeks, airplanes need what is known as the A Check. In this process, technicians change filters, check, and lubricate critical systems. They also give a detailed inspection of all the emergency equipment. The A Check takes between six and 24 hours on a narrowbody aircraft. 

the A Check is performed approximately every 400 to 600 hours or every 200 to 300 flights. 

B Checks

B Checks are only used on some aircraft, but are mostly cabined into other checks now unless needed.  The B Check used to be made every six to eight months. It took up to 180 labor hours and could be completed within one to three days at an airport hangar. 

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C Checks and D Checks

The heavy maintenance is much more extensive than A and B Checks. The C and D Checks take out the aircraft of commercial services for several weeks.

C Checks happens every 18 months to two years. It takes three weeks.

A C Check takes up to 6,000 maintenance hours. Technicians perform specific tasks such as examination of structures and in-depth lubrication of fittings and cables. 

D Checks are the heaviest maintenance visit of them all. 

It occurs every six to ten years, and it involves comprehensive inspections and repairs of the entire aircraft. Technicians basically dismantle the airplane and put it back together.

Everything in the cabin is taken out (seats, toilets, galleys, overhead bins) so engineers can inspect the metal skin of the aircraft, inside out. The engines are taken off.

Also, they remove the landing gear and aircraft systems, which are checked, repaired or replaced, and reinstalled. 

The D Checks take about three to six weeks and costs several millions of dollars. Once it is done, the plane is almost like a brand new aircraft. However, when an aircraft receives between two or three D Checks, it may become more costly than the plane’s actual value. Then, it is time to retire the aircraft (or resell it).