How long should a patient who overdosed on opiates and required narcan for respiratory resuscitation be observed in the ED prior to discharge?

About 5 hours.

Half life of narcan is 1 hour. At five half-lives, or 5 hours, the initial dose of narcan is 97% eliminated and if the patient has not required another dose of narcan for recurrent respiratory depression by this point, it is probably safe to discharge the patient home.

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Source

Naloxone: Drug information. Lexi-Comp

Image source: http://www.nes.scot.nhs.uk/prescribing/topics/TDM/fig2.gif