Well it all depends on how much vibration you have to deal with.
Most of the wear on the servo gears happens on the two gears closest to the output shaft. The plastic gear on the bottom is seldom effected. Except when the servo is overstressed the plastic is the first to break.
My experience is with jr 8411's . A good practice is to replace the gears once every season on the surfaces that work the most and suffer from alot of vibration.