I notice a slight blunting of my mental edge at about 1000ppm. I always maintain enough ventilation to keep below about 800ppm. I don't guess; I monitor.
At work, in our delapidated office, I've seen up to 1800ppm by lunchtime. No wonder no one can concentrate in there.
It's not cheap, but you can buy heat-recovery ventilation for domestic properties: stale air is pumped out, fresh air is pumped in, and there's a heat-exchanger to warm the incoming air. Typically, air is extracted from rooms such as kitchens, bathrooms and toilets (so that you expel moisture, smoke and odours) and pumped into living spaces such as lounges, dining rooms and bedrooms. It doesn't provide enough air changes per hour to mitigate the danger from Covid-19, but it does do enough to keep CO2 levels down where you want them, and it avoids arguments with people who want to close all the windows and doors because they "don't like a draught".