If you keep your planes flying a lot they will need a D-Check at 5,000 flying hours. A D-Check is very extensive and expensive, but very important! Neglecting to perform D-Checks may result in an incident or even a crash. 

D-Check Progress and time remaining

D-Checks take your airplane off the line for 74 hours. Thats over three days out-of-service But after 5,000 hours of flying both the plane and you need to take a break. 

Up to this point in the game I am willing to bet you have not had an airplane crash. Even skipping C-Checks does not result in a crash (correct me if I am wrong). So store up your cash and preform the maintenance as it presents itself. 

When you preform your first D-Check you will get the "D-Check" Achievement. The D-Check Achievement I earned today is pictured below.

In summary; you may be playing way to much if you have planes getting D-Checks. 

As far as I know you cannot see how many hours your plane has flown. One reason you may want to know is so that at 4,999 hours you could sell your plane and get a new one. That's okay with me because it's likely that you would loose money in the transaction. Selling a high hour plane for a low price and buying a low hour plane for a high price is not a wealth building business model. 

Link here If you would like see all the Airline Manager strategy: Facebook Airline Manager Resource Page