i try to replace the gear set and bearings at least every few seasons or when they show signs of wear. taking the servo out and using your hand to manually spin it from stop to stop is about the best way to check for mechanical problems, on the electrical side you pretty much have to have a feel for the bird and know when it's acting different. sometimes the servos will give a visual indication (jittering) that there is an electrical problem and you can replace it then or send it in for a check over.

I've had servos last 10 yrs and I've had some last a few months both in ideal conditions so there is no set time frame.

kc