How long does it take for a given concentration of carbon monoxide to kill you? Insofar as it has considered this question, the holocaust and anti-holocaust literature seems to have followed F.P. Berg in suggesting an exposure of 0.4% as the minimum necessary to kill in a 30 minute exposure (which means gassing time greater than 30 minutes due to the time it takes to fill the chamber with exhaust). Berg also noted that it might be necessary to use as much as 0.8% to kill reliably.
There have been, to my knowledge, only two experiments on humans that address CO concentrations in this approximate range. The first is a study by Chiodi:
Apparently his experimental subjects endured exposures as high as 0.35% for 70 minutes without severe consequences.
Next of interest is an experiment by Haldane.
These tables describe Haldane’s results, and pretty much speak for themselves.
Taken together, these studies certainly show that carbon monoxide concentrations of 0.4% for 30 minutes would not have been enough for use in gas chambers.
Finally, here is some animal data:
(source for all of the above: Acute Exposure Guideline Levels for Selected Airborne Chemicals: Volume 8)