I live in Germany, and here the houses generally don't have machine ventilation so one has to air manually and regularly. As an example: earlier today I closed the door to my office (ca. 18 m2 / 194 sqft). The window was closed. After 2 hours the CO2 levels rose from 935 ppm to 1966 ppm. Letting air in for 20 minutes reduced the CO2 levels to 869 ppm.