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Do you:
1. wait until you can feel play in the frame
2. replace them according to some fixed schedule
3. regularly remove the shock and check for roughness/play
4. none of the above
For me a full strip down, so you can at least feel each individual bearing for any roughness which might be missed when its all bolted together.
Push your finger into the center of the bearing to feel any roughness.
Suspension bearings only move a few degrees compared to wheel bearings.
A mixture of 1 and 3 - whenever the shock is off (I seem to get through shock bushings at an alarming rate), everything gets a once over and any roughness investigated.
If I replace one of the bearings (so have removed the pivots etc), I'll pop off the seals and pack clean grease in the other bearings. I generally wait until there's a reason to dive in and start playing with things though - I'm paranoid about rounding out the bolts on my suspension (alloy allen bolts) - so don't remove more than I have to.
when they start creaking i.e. when the races are starting to break up