Service when they need it.
Go ride in the alps, in the wet, for a couple of days. Suddenly *everything* needs a service, as I found! 😉
Even my plush forks that have never had a hint of needing a service in 2 years (despite the manual saying every 20, 50 hours for this, 100 hours for that)… seriously needed a service after biblical mud and rain.
One short alpine trip – Needed service: forks, BB, headset, Reverb, hub and/or lower link bearings (yet to find out). Not to mention how much it went through pads plush a nice chunky hole in a brand new tyre.
But yeah, I won’t strip down and service a fork if it doesn’t feel like it needs it. Though maybe not wise. My hard tail fork which is 4 years old, never serviced and got to the point where it feels like it really needed it, so I serviced it and found the air shock has cracks in it. Had I serviced it earlier, maybe it would be okay. Then again, maybe not.