The Rock Shox on my MTB is over 20 years old, has never had a thing done to it, works fine. I guess I top up the air every couple of years. A suspension fork, mounted on a bike used mostly on the street or mild dirt trails, is doing very little - occasionally compressing an inch or so, but most of the time nearly rigid. No reason to do anything to them until they give you trouble. They probably never will.
Now that's the kind of advice I like! Do nothing! I wondered how important it was, because I doubt these bikes ever saw any trail time what so ever. Thanks!