Question

what would be considered an acceptable amount of mileage that could be run with a P0300 showing

Answer 1

P0300 Random misfire

This code is a Type A code, this mean it is catalytic damaging, The whole purpose of the CK ENG LT is  to protect the catalytic convertor. The catalytic convertor is to protect the environment from harmful emission into the atmosphere. There is no acceptable mileage,  even at idle you can do harm to the catalytic to point where it either useless or may internal damage