This is an interesting question, one that I have pondered for years.
So, I am running a long term test. I have a BoBcat which is almost 10 years old with 300 flights on it. The rudders have the original JR9411 servos which are gyro activated so they are active, almost all of the time, lots of stop -starts, hardest on the motor and amplifier. As the BobCat has twin rudders a single failure will be almost inconsequential. So far they continue to work perfectly with no increase in gear play etc so the servos will remain in position until they show signs if degradation or failure. So far so good. They may even outlive me !

Regards,

David Gladwin.