There are many cases where the level does not drop to ambient levels, e.g. offices in basements, low-energy houses etc.
We even saw schools where the classrooms were not able to drop to ambient levels over the whole unoccupied weekend because the ppm levels on Friday afternoon were beyond 3000ppm.
The automatic baseline calibration period of some of the NDIR CO2 sensors can actually be adjusted through the software. I believe the Aranet allows that and also we do have that feature on the AirGradient dashboard. So you can switch it to a much longer period or switch it off completely.
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).
We even saw schools where the classrooms were not able to drop to ambient levels over the whole unoccupied weekend because the ppm levels on Friday afternoon were beyond 3000ppm.
The automatic baseline calibration period of some of the NDIR CO2 sensors can actually be adjusted through the software. I believe the Aranet allows that and also we do have that feature on the AirGradient dashboard. So you can switch it to a much longer period or switch it off completely.