Well, to be perfectly accurate, you can't.
PPM is a dimensionless quantity while mg/hr has dimensions of mass/time. You're going to have to multiply it by a period of time and divide it by some mass to achieve the correct dimensions. Fortunately, we can make a few measurements and come up with these quantities.
I'll start with the mass, since it's the hardest to compute. In 1 hr, you produce 1200 mg of Ozone. In order to find ppm, I need to know the mass of the air into which these 1200 mg of Ozone will be spread. This is where the measurements come in. Let's say the room is 4x5x2.5=50 m3
. The mass of the air in the room will be it's Volume x Density = 50 m3
X 1.25 g/l x 1000 l/m3
x 1000 mg/g = 62,500,000 mg.
If you divide the mass of the ozone by the mass of the air, you'll get the mass fraction of ozone. Multiply that number by 1 million, and you've got ppm/hr: 1200/62,500,000*1,000,000=19.2 ppm/hr.
Now all you've got to do is multiply that number by the time you run the machine and you've got ppm. For example, after 1 hour you'll have 19200 ppm/hr X 1 hr = 19.2 ppm.