Don't know about the 8611. My experience with 30 or so 8411's is :

o airframe vibration is what wears them out
o a big, heavy, foam wing airframe with a very smooth twin (like the brison 6.4) wears them out more slowly
o most wear happens around idle settings
o 8411's last a lot longer on vertical surfaces (rudder) than horizontal ones (elevator, aileron)


The life cycle of my horizontal surface 8411's (I've been through 30 or so) has been :

initial install : great
20-50 flights : slop is greater. Can't feel it flying but can with my fingers and can see it in greater surface vibrations
80-150 flights : pots start to wear out from all the small oscillations (whether you've replaced the gears or not).
At this point your surfaces may begin oscillating on their own on the ground.
150+ : By now you've pulled the servos, sent them to horizon, and they've sent you replacements. Excellent
service, usually replace rather than repair.

My experience has been exactly the same with metal (8411) and plastic (8411sa) gears.