The "regular" meter to use is lower-right, next to the Speed Telegraph, and is the CO2 level, which displaces the O2 - that's the one to be warned about.
The "warning" will show in the upper right corner, where you see the "New Objective" icon in that image. Both will display the amount of similated CO2 / O2 in the simulated atmosphere. The warning will usually after just a few hours under, but you are generally good for 24 hours or more, dependent upon the boat and conditions. The "warning" icon will change color as time advances and the CO2 level increases. I just tried to find stats on CO2 levels, but all I got were hits for climate change, in spite of being very specific with my search terms... but there is a formula somewhere... perhaps someone remembers the level a human can withstand short-term before the are rendered unconscious... The different members of your crew will react differently, which is a random function of the game.