Unfortunately, there is no easy way to compare mg/hr and ppm. Milligrams per hour is a measure of the molecular weight of the ozone produced by an ozonator in a one hour period of time. Parts per million is a volumetric measurement of the number of ozone molecules in a million equal parts of air. They do not directly correlate except in an ideal lab situation where the test area is of a known volume, the test area is sterile (nothing to oxidize) the temperature is constant and air movement in the test area controlled. Only then would one be able to correlate the two where X mg/hr produced XY ppm.
In a real world you may be able to smell the ozone from a machine producing 20mg in a room that has relatively clean air, but the same 20 mg in a cigarette-filled smoky room would not even be noticed until it had eliminated the smoke chemicals in the air.
20 mg/hr does not sound like much, but over time it is plenty enough to remove even serious odors in 12X12 room.