Makes sense Cav, on my last motor they didn’t let the pad wear on the menu get below 25% before swapping out, changed at mileages a little less than those i noted above but exhaust brake not up to current standards, plus vehicle driven hard and on the brakes constantly* by others when i was off shift as is standard car driving method of todays lorry drivers 
, you can tell how its been driven by the brake dust mm thick on the wheels.
El presidente’s Volvo has a good retarder by comparison so be interesting to see just how long he can make them last.
*We had one bod who always turned the auto exhaust brake off and would only slow on the brakes because he didn’t like how it changed down by itself with the exhauster in auto mode, you couldn’t make it up.