I can't speak for what 40% and quitting means for the regular person but I can tell you that among trained distance runners they want to quit soon after they hit OBLA (onset of blood lactate) and ventilatory threshold during treadmill tests. This occurs to them at somewhere between 80% and 92% of VO2 max. And is almost always accompanied by a Borg scale of perceived exertion number of 16 or higher.
At this time their ventilatory volumes skyrocket and if you are taking blood their lactate which may have been steady at 4MM will start going up to 8 MM rather quickly and up from there.
And we see this in everybody. The main difference is the better the runner the further along they are in the treadmill protocol
40% of their max effort on a treadmill is the equivalent of a light to moderate training run.