By how much will the oxygen in a room be depleted by carbon dioxide?

A room has a volume of 30 cubic metres and the air change rate is 1 per hour. A gas fire is spilling carbon dioxide (as well as a tiny amount of carbon monoxide) into the room at a rate of 1.343 cubic metres per hour. I am trying to find out by how much the oxygen (uming 20.9% in air) will deplete, due to the carbon dioxide, after n hours. I need to know if the oxygen will go down from 20.9% to 18.5% and if so, in how many hours. The problem is more difficult than I at first imagined. Any help that you can give me would be much appreciated.