Historically, the 12 inch round magnet brakes on a tandem axle, normal towing, last about 18000 to 20000 miles.
The oval magnet brakes last a little long, about 20000 to 22000 miles, again average.
The magnets are always the first things that wear out.
The shoes will last 3 to sometimes 4 times as long as the magnets.
Again these are averages.
When magnets wear beyond their design limit, they can start scoring the armature face of the drum, as well as you risk getting into the magnet coil of wire. When that happens, brakes will short out and depending on how your rig is wired, the brakes are gone.
Not every brake shop can turn the armature plate faces of the drums, so you usually must find a machine shop.