That's not adequate, at least for me as a relatively average sized man in a relatively average sized living unit. My respiration is sufficient to raise CO2 levels indoors to over 800ppm from background levels within an hour without constant ventilation.

No it's not.

CO2 levels build up higher than I want within half an hour so I'd have to be doing that all day long. And it would result in a periodically freezing apartment.

It's much more efficient for me to keep it open a crack.

Do you have central air, and keep the fan running? I've experimented with this at our place and doing a full cycle for 5 minutes, then turning the fan to "on" rather than "auto" and closing all doors and windows will keep it below 700 everywhere for about 6 hours. Otherwise the second floor climbs into the 1000's while the first floor stays around 500

I mean efficient in terms of heating/energy, not CO2 levels.

I got the window unit version of https://www.openerv.org and it is better than leaving the window open.