Every year I pull the hubs and inspect / clean the brakes. Every other year I replace their inner side seals. IMO, self brake inspections takes very little time with blocks, bottle jacks and axle stands (as safety jacks) and is a very easy DIY task.
Remember that a sitting (less traveled) trailer allows rust to grow on the inner / hidden side of the brake drum. When the drums do move, its like taking a sharp file to the brake pads. Ouch!!! Thus.... A trailer less traveled needs more brake inspection intervals. re: 2 x year instead of annual. Especially if one lives in regions that promote rust - like in damp and/or coastal regions.