How long brakes last will vary greatly from customer to customer depending on how they adjust their brake controller (one reason we really love the Autowbrake brake controller) and how many miles of interstate driving with little braking per mile.
If I had to put a number on it based on miles I would say 75K miles.
Keep in mind too with trailer brakes when they need service it is not a case of throwing new shoes in it, the whole brake assembly is replaced as a complete unit.
As Sweeney indicated, we at Camp-Inn do not trust "trailer shops" to do this type of work, especially wheel bearings. However, if you are a point that the brake units need replacing you might have to go to a RV place because the auto shops might not have experience with these trailer brake assemblies. The auto shops are still the best choice though for wheel bearing work.
Cary