VW, There are many variable's that can cause a tire to be changed. the tires on a 747 are usually 49 ply tires and can take quite abit of abuse. compare that to the 2 or 3 ply tires on your car. tires are required to be changed usually when the ply cord is visible. this happens when a tire hits the runway on landing. the tire is not spinning and when the tire hits the runway at 120 mph or better it skids until it catches up with the speed of the plane (hence the rubber on the runway that you mentioned). many times tires run over foriegn objects when on the runway or when taxiing, tires have a maximum cut depth limit then they must be changed....usually5/8" depending on the tire wear. another reason to change tires is pilot's. some pilots have lead feet when it comes to applying brakes and can wear a tire down in a hurry. there are no set amount of landings for changing a tire. as a foot note ...us mechanics really would rather not change tires if we had a choice (which we don't) , a wheel and tire on a 747 can weigh around 250 - 300 pounds and they take quite abit of brute force to change. one of the worst nightmares a 747 mechanic could have is for the aircraft to land and the anti-skid fails, many times this will blow many tires at once. the 747 has 16 main gear tires and I have had to change all 16 at once do to an anti-skid failure...not a fun thing.