For how long should a patient stay on oxygen after being on nitrous oxide?

Not Long: There is a theoretical concern that after discontinuing the inhalation of nitrous oxide, the rapid transfer of the gas from the body into the lungs to be exhaled, could displace oxygen from the lung. Breathing oxygen for a few minutes should prevent this. Once a patient is awake and breathing normally, there is little to no risk.

Several minutes: In my office, when nitrous is administered i always have the patient breath pure oxygen for some time after once the nitrous is turned off. We wan to clear out all the nitrous from the lungs so that patient can leave refreshed and clear headed.