Once a year is probably enough if you aren't running the wheels off of stuff. I give rollers one drop on each end of the roller pin when new and only oil them afterwards if theres an obvious squeak. Steam linkage probably requires slightly more frequent oiling, I usually touch a finger to the rotating points and if theres a dot of oil on my fingertip I'm good.
The best tip I can give is to use labelle 102, this is a "heavier" light oil and stays much longer where its put, instead of running off over time. This helps to greatly reduce reoiling intervals.
