Many will tell you once a year. Now think about that for a second. How often do you repack the front bearings on your car. They are exposed to a much harder condition, turning.
The more important thing and maybe the reason for once a year is the inspection of the brake magnets. Depending on your driving, lost of hill and dale or lost of turnpike, you should set up your inspection interval based on that. My utility trailer dose not have brakes and the wheel have not been off in at least 30 years that I know of, and most likely a lot more than that.
I inspect my brakes every other year because I use the trailer more than the average.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles