For those with air brakes, how often are you rebuilding your air dryers? Our rule of thumb is every other year, but some of our fleet will put 6000-8000 miles in a year, while others put 16000-20000 miles a year. Obviously the ones with higher mileage see a heavier amount of use and become saturated faster. My question is what does anyone else base this off of? Calendar time, mileage, hours?