Diminishing suspension performance is subtle; deterioration is gradual so most riders don't notice it happening.
The most critical part of your suspension maintenance is the oil. When it's fresh, the damping works at its optimum; it will lubricate the suspension internals and will maintain its damping characteristics as things heat up or cool down.
The thing that "wears oil out" from a damping point of view is heat. Heat causes the long molecules of an oil to break into small molecules. Oxygen will react with oil to create heavier particles which become sludge that clogs narrow channels. The oil's job is to convert energy from bumps etc. into heat, therefore it has a limited life. Moisture will find its way in, no matter what you do to stop it. This contaminates the oil. And lastly, the action of the suspension, one part sliding on the other, will produce wear particles. This also contaminates oil and reduces its effectiveness.
To keep suspension working at its best means:
Keeping forks and shocks clean
Keeping parts clean will extend the life of both forks and shocks; don't leave your suspension dirty as grit will cause damage to chrome surfaces and seals, reducing their life. Cleaning under external scrapers or wipers, especially on forks will help lengthen the seal life. Backing this up with a sponge ring soaked in oil will add another barrier and lubricate the external scraper and the inbuilt scraper on the fork seal extending its life. Keep pressure washers away from any seal
Checking for worn out parts
Any parts that are excessively worn will cause a lack of precision and control from any suspension system. Worn fork bushes mean you will not have precise steering. Worn linkages mean free play at the wheel that is uncontrolled suspension action and this can cause loss of traction. Remember, there's a lever ratio that multiplies any clearance in any of the linkages; 0.5mm wear at the shock pivot can stack up via lever ratios and bearing internal clearances to be 4mm or 5mm at the wheel, resulting in loss of traction.
Metal parts move against each other creating friction and wear. The higher the oil viscocity, the more it will protect moving parts. Suspension damping, because of the hydraulic nature of its design, requires oils of low viscocity and, together with the additional demands made on it, should be changed more regularly than other oils to protect parts from wear.
Regular service intervals and oil changes
Maintenance Periods and Oil Change frequency
The best way to understand when you should change your suspension fluid is to recognise that the oil's job is to convert the energy from bumps and braking etc into heat.
After oil is changed, within a 50km ride oil will become discoloured although its performance will not be degraded. It is the change in damping characteristics of the oil that will most alert you to the need for a service; the oil will gradually lose its viscocity and its damping effects will be reduced. The main thing that causes the oil to lose viscocity is heat. So if you put a lot of heat in over a very short period of time e.g. a Superbike with a day's practice at near lap record times then you'd expect to change it after every practice. We recognise this is not practical for the road or dirt or most enduro or ride day applications...
So as a guide for a roadbike you should change the oil in forks every 12 months or 15,000 to 20,000 km. The scrubbing action of forks is the main problem. Dirtbikes are a little harder to give a guideline for but if you use 40 to 60 hours of use (providing you're not too full-on) will work.
A shock is a different matter, but generally once you have your shock set-up and you find you need to change that set-up, then you need to change the oil. If you use the shock in any Ride day or Dirt competition the time between services will get shorter, how much shorter depends on how hard you work the shock. But if you pay attention to how the shock works and, of course, how quickly it starts to fade, then you will definitely know when to service the shock.
Untouched suspension at 40 to 60,000k? - the oil is putrid and smells like distilled cat's pee. Generally the suspension internals are then so bad that other parts need to be replaced.