The rotor rotates up to 5000 cycles per second. Thats why the durability significantly depends on the rate of use and how you use it. As a result of that it´s not possible to give a clear statement about how long the rotor will keep working but there defintely some good advises you should follow.
No grinding pressure! No grinding pressure! No grinding pressure!
Grinders have to snap in the chuck!
Don´t forget to grease!
The water nozzle always has to be free! Prevent plugging.