I see various exposure limits for carbon monoxide, but what about one breath?
"In any high level CO environment, your exhaled breath CO level will gradually rise but remain lower than the ambient level because you always absorb some of the CO you inhale as long as the level in air is higher than the level in your blood.  As soon as you stop smoking or move to a lower CO environment, you start exhaling."

So I suspect a lungful of pure CO would indeed make you feel awful, but since that wouldn't be nearly enough to replace all the oxygen bound to your haemoglobin, I imagine it wouldn't be fatal in a healthy individual, especially if you were breathing air free of CO after that one lungful. 

Apparently it takes 5 hours to eliminate half of your blood CO also.