I rotate all my 26 locomotives in about a two-three year cycle. I place a small sticky inside the box to indicate what, if anything, the items needs for maintenance or replacement/repair, and what I did recently if it should last for a couple of years or more. In my case, I do tend to lube the outer works, any rods and levers, and the driver axle boxes, plus the sliding surfaces on the trucks where they pivot. When I open the box a couple of years later, I know what I must do and what I can assume is still in good order.
So, more specifically, about every two years for me, but that amounts to about 2 hours of total running over the couple of months or so that the engine gets run on the cycle.
If you are interested, the hobby has many good/great lubes. But, someone left me a plastic container of Dextron III Mercon auto transmission fluid, which I know is safe on plastics and on paints. It's a superior synthetic fluid, so I have been using it for about 15 years. So far, nothing to complain about. Seems to work just fine.
For an applicator, cut a 2" length of dowel, maybe 3/8" or 1/4", and insert the pointed end of a darning pin into the end of the dowel. Next, using side cutters, cut off the end of the eye of the needle. The idea is to leave two parallel tines. Use this to dip into your lube of choice and touch the laden tip to whatever you wish to lubricate. Works like a charm. An Opti-Visor really helps.