I have seen on the Internet that you should grease large travel trailers once a year or every 10,000-12,000 miles and small trailers every 2,000 miles.  We make about three, 2000 mile trips, partly on Interstates, a year with our 16-ft.  I grease the bearings before each trip.  If we were just making many short trips, I would probably just grease it each season.