When they stop flipping or are losing power.

I usually go thru them when shopping a game out and rebuild if they are getting sloppy.

I would say like the tires on a car, you can inspect the parts for wear from time to time and replace when needed. Or you can wait for a flipper blowout and replace then.

At least you don't have to fix them on the side of the road.

In my experience replacing coil sleeves is usually good enough in the majority of cases, as well as cleaning off the grease and metal shavings from the mechs. If that isn't enough buy the rebuild kit.

IMHO when the stops are visibly indented or the plunger ends visibly mushroomed or stiff in the sleeves it's time to replace parts beyond the coil sleeves. Usually you can go through a couple of coil sleeves before that happens, but the easiest thing to do is remove the coil stops and look at both them and the plunger ends, plus check for slop in the links.

I just replaced flipper plungers, sleeves and stops on a friends Lotr. He was getting buzzing when holding button. Think I narrowed it down to the slop in the plastic pinion that mounts to flipper mech. So I change them when I lose power, feel slop or annoying buzzing