Mountain bike suspension is high quality stuff, but it needs servicing on a regular basis to stay in premium condition. Service intervals are different from product to product, and are dependant on your riding conditions, but as a general guide, your suspension should be serviced as follows:
Dust Wiper Service – 30 to 40hrs
Damper Cartridge Service – 100hrs
Air Sleeves – every 30hrs
Dampers – every 100hrs or annually.
Every 100hrs, or as required.
Here's a couple great example of some MTB suspension that is worn out, due to lack of servicing.