About every 500 flying hours the commercial plane goes through an A check which is just general service such as changing oil, checking tires, making sure the engine is in good shape and working properly, checking the controls etc.
Then a more extensive test and service is done every 6 to 10 years which goes over the entire plane and sometimes by then the engines on the commercial airline need rebuilt or replaced.
As well as other mechanical parts and sometimes if the plane is 20 years old or older and it comes to a D check the airline may decide to scrap the plane instead of repairing it because of both safety issues as well as the time and cost it would take to perform those extensive repairs that are likely needed.
Some airplanes go as long as 40 years and continue to get the repairs done on them and have those D checks every 6 to 10 of those years.
In my opinion though airplanes that are commercial airplanes flying passengers are not really serviced enough or checked out enough.