It's not just mileage. There are a handful of events (key on/off cycles, gear changes, etc.) that have to happen to complete the various monitors. Case in point: Girlfriend had an '02 Montero Sport with a persistent CEL so it wouldn't pass inspection. I replaced a few cheap possibilities, cleared the codes, then ran it on jackstands with a stick on the pedal to hold the speed around 70MPH while I did other things. After about 75 miles, the light still hadn't come on. I set it on the ground to take it around the block to ensure. Just as I had made the turn to come back home, that damned CEL came back on for the same code.