Service Intervals - Forks & Shocks
Every manufacturer has a different service interval and method. Every rider also has different conditions and intensity which make it more complex, but here's a very simple method that works for any fork or shock to maximise performance and minimise wear.
The wiper seals on forks and shocks are designed to leave a very fine oil/grease sheen on the fork stanchions or shock damper body. This gives the stanchions/body a shiny appearance.
If your stanchions or shock body look dull, then they're dry and need serviced. Internally it means the reservoir of oil or grease has either been used up (it spreads to the outside and dries off) or displaced to part of the fork/shock where it can't lubricate any more.
If you're unsure, simply clean the stanchions/body with a dry cloth and pump it up and down a few times. If it stays dry then it's service time. If the sheen comes back then you're okay.