It takes millions of years to change. The amount of O2 in the atmosphere is large compared to the amount of organic carbon in the biosphere and the available fossil fuels, so burning the biosphere (trees mostly) and fossil fuels don't change the amount of O2 that much.

Over millions of years, oxygen oxidize metals and carbons brought by volcanoes and then exposed through erosion, so oxygen level can go down. This is opposed by having living beings producing organic carbon-rich sediments that becomes rocks under water. Though a small percentage of those rocks are coal, most of it don't contain enough carbon to be fossil fuel or are too deep under ground.

Over millions of year the oxygen level change. It is believed it peaked at some 30% in earlier times. Also, it has been close to 0% for billion of years (it took a long time for oxygen produced by life to oxidize all the already available metals, mostly iron).

The amount of CO2 changes fast because there is very little CO2 naturally in the atmosphere. That's because CO2 is very soluble in water and also combine easily with ions such as calcium ions to make carbonated rocks. So over million of years, CO2 in the atmosphere doesn't change much even though O2 changes a lot. If you include the CO2 in the ocean and the carbonated rocks then it changes a lot. That's also why we say that the CO2 we put in the atmosphere will last about 100 years. If we were to stop putting more CO2 in the atmosphere, it would go down to its previous levels by the ocean absorbing the CO2 in about 100 years.