Agree with "ga traveler" ...there is a very long interval between the necessity to change brake pads or shoes under normal conditions with everythng working up to specification.
It is very dependent upon the type of road conditions ...one who travels mostly in the flatlands will get much more total mileage than one who travels mostly in mountainous areas. City driving will wear them out quicker than long-haul... naturally.
Having said that, there is no absolute mileage formula, but rather an expectation that they should be checked by a qualified brake technician at intervals which are dependent upon type of service. An overall safety inspection prior to any long trip would be a given ...brake pads are just one part of a complex system which must work perfectly.
Jim and Fran in Central Florida
1989 Country Coach Sedona Mark V
Turbocharged Detroit Diesel 8.2L "Fuel Pincher"