The normal half-life of carboxyhemoglobin, when the patient is breathing room air, is 240 to 360 minutes (4-6 hours). The half-life of COHb can be decreased to 80 minutes with the administration of 100% oxygen.
It can be brought down even faster with hyperbaric oxygen therapy. Hyperbaric chambers are reasonably common, due to usage in both (SCUBA) diving related medicine and various therapies for wound care.
According to the toxicity curve from wikipedia, even at 2.5 bar, you wouldn't start to see issues until ~4 hours have past. That's enough time to remove quite a bit of CO.
