I have seen this question debated and thought I’d ask the 'dope. Historically, atmospheric CO2 levels were less than 300ppm. These days the level is up above 400ppm. At ______ppm, people start to complain that the air ‘isn’t fresh’. At ______ppm people feel drowsy or function poorly. At ______ppm people start to die.
You probably have a long time to worry, but there are actually answers to these questions, in part, because CO2 is an easy to use measure for indoor air quality (if you think about it - with port ventilation - you’d start to fill up to exhalation standards of CO2). Luckily it seems like you have to go way, way up there to start to effect health. I think OSHA sets a limit of 5,000 and I make no effort whatsoever to ventilate my place (windows are never open) - and I actually have a CO2 meter - and although I can breathe on it and make it go up over 1000, it never goes naturally above 600-700.
Of course by long time to worry I mean directly impacting your breathing - I think by the time that happens all the glaciers will be melted and other indirect (if you can call it that) effects will cause massive problems before then.