|Hi Bruce, welcome!
The answer is not that easy, it really depends on the fan. For example, most Emerson fans hold a significant amount of oil, but by contrast, Eskimo and other dime store fans hold very little and need to be oiled more regularly.
There generally is a wick of some sort, whether accessible or not, that holds an oil reserve. Yes, many fans that have a greased oscillator box, do not have an oil cup or other means of oiling. They relied on the grease breaking down and giving up some oil to lubricate the shaft. Personally, I like to oil that bearing, especially when reassembling a newly restored fan. Just don’t oil too much or the oscillator box will begin to fill with the excess oil.
As far as how often. Again, it depends on the fan. If it depends on wick oilers, probably running it 7 hours a day, every day, I’d oil it every couple months, or if it begins to get noisy or run slower than usual. Emersons have a large reservoir and their advertising bragged that their fans only needed oil once a year.
Post a pic of your fan and we can give you better suggestions.