When does narcan start to work in the body is it immediate? and how long does it last in the body?

When Narcan is administered intravenously (I.V.), the onset of action is generally apparent within two minutes. The onset of action is slightly less rapid when it is administered subcutaneously (S.C.) or intramuscularly (I.M.). The duration of action is dependent upon the dose and route of administration of Narcan. Intramuscular administration produces a more prolonged effect than intravenous administration. Since the duration of action of Narcan may be shorter than that of some opiates, the effects of the opiate may return as the effects of Narcan dissipates.

http://www.drugs.com/pro/narcan.html