For vintage style caged bearings depends on mileage and environment. If the bike is a new acquisition its a good idea to service the bearings to establish a base line and ensure the bearing balls and races are healthy, plus set optimum pre-load. I do my vintage bike about once a year, primarily because bearing pre-load increases on this BB during use, not so much due to contamination or lube loss; this bike never sees rain, I don't wet it when cleaning, and it is rarely used on unpaved roads. If I were to ride in the rain or use a water hose to wash it, I would shorten the service interval.
I service (re-lube) my UT external cup Campy crank bearings around 1500 miles. The interval on this bike represents around 50 miles of rain riding, and 200 miles of un-paved road, in addition to clean & dry conditions.
Sealed bearing BBs can go a long time without service; some BB types with inaccessible bearings are non-serviceable.