curious - what fork and shock are you running? Do you just take it to the LBS or remove them yourself and send to someone like suspension experts, dirt labs, etc?
I should have mentioned my intervals are for coil fork and shock. Air shocks need more frequent sleeve oil maintenance, about once every 3 to 6 months to avoid drying out (or possibly longer, depending on frequency of use), about the same interval as any air or coil fork needs the oil changed. Like coil an air shock damper oil service interval can be 1.5 years more or less to maintain best performance.
I've found these intervals pretty consistent for preventative maintenance over 13 years of full suspension and another 5 years earlier on hardtail suspenion. I have gone much longer when I didn't know any better. The deterioration of performance is gradual and not very noticeable really until maintenance is done too late bringing back the new bike feel.
I have always had just one versatile trail bike at a time. During the last 3.5 years it has been an Ibis Mojo, now with a Rockshox Lyrik coil u-turn and early (2002) Fox Vanilla RC that has been custom tuned twice by PUSH Industries.
I do my own fork maintenance, except if bushing replacement requires special tools. Many years ago when I used air shocks I'd service the sleeve myself, which is easy. Most forks are easy to self service if you have the few basic tools required and follow the service manual. Shocks require expert damper oil change service, I hope to learn to do this myself to gain more knowledge and save money.
If you are moderately mechanically inclined, you would enjoy doing your own fork and shock air sleeve maintenance, especially after doing it a couple times to learn it. The fork oil maintenance can be done without removing the fork from the bike. The shock must be removed to remove the air sleeve for cleaning the old oil out and replacing with fresh. The fork can be done in about 1/2 hour without rushing, and the air shock sleeve oil change in less time.