This question was asked here not long ago and I believe the consensus was that the servos in a jet have such a low level of activity that the gears, motors, and pots don't wear very much. Most guys echoed the sentiment above - don't replace them unless they show signs of wear. Now in a 40% 3D airplane or helicopter, I think the answer is different and there the top guys generally replace/rebuild their servos every couple of seasons - at least the ones I've talked to do...

Bob