Round magnets should be replaced when the "top" of the three screw heads start to wear. Should the magnets not be replaced at that time, and if the magnet is wearing unevenly, and they usually do, you will take the chance of going into the wire coil, which means an instant short in your brake system.
Typical all around mileage expectations for round magnets are for a single axle, about 15,000 miles. For tandem axle trailers about 18,000 to 20,000 miles. Shocks should be replaced at every other magnet replacement. Brake shoes on single axle trailers usually last about 40,000 miles, and on a tandem, about 50,000 to 60,000 miles. A bearing repack should be done once a year, or, every 10,000 miles, whichever comes first. Grease seals should be replaced at every bearing pack. Running gear should be balanced every 10,000 miles.