How long a motor last depends on how you use it and how well you take care of it. If you run it rich while flying and you put in some maintenance of the motor after flying it should last you for a long time. I learned to fly 9 years ago one a trainer that had an OS 40 FP motor on it and it's still running today, the motor has well over 500 flights on it. A friend of mine at the field has an OS 91 4-stroke that was close to 15 years old and too many flights to count, only major maintenance to date was replacing the bearings this year. But if you run the motor wide open and leaned out you can pretty much assure that it won't last long.

Hope this helps

Ken