It depends a lot on the outdoor airflow situation. I live in a 6th floor apartment in a windy area, and cracking a window for 2 minutes gets me below 500 ppm ev

It depends a lot on the outdoor airflow situation. I live in a 6th floor apartment in a windy area, and cracking a window for 2 minutes gets me below 500 ppm every time. At my parents’ house, I have to leave multiple windows fully open for hours to get the CO2 reading to budge at all. The air is just too stagnant. Things improve a lot when I put a fan in front of one of the windows to blow air out. (Another problem is that their HVAC system is always recirculating the air, so the room I’m trying to ventilate is continually getting replacement air from a 1500 ppm wing of the house).