It always comes down to how well you can ventilate. If you can open windows / doors to different sides of a house / building there is usually some natural air f

It always comes down to how well you can ventilate. If you can open windows / doors to different sides of a house / building there is usually some natural air flow, and the stronger it is, the faster it will bring CO2 levels down. In the apartment here it can be less than 5 minutes to have CO2 levels to ambient levels, but it also takes longer if the air is "standing" even outside. For that you will need active ventilation then, clearly, same for having windows to the same side of the building only etc. There is also another thing: In multi-tenant buildings you may have others airing out their apartments which means you will not get ambient levels, but elevated ones due to added CO2 from the other apartment(s). For example, I see >500 ppm CO2 then. Also, even the ambient levels are not always the same; I have seen values between 416 and 432 ppm depending on the day, wind, and other factors (living in a larger city).