Q: How long the effect of Naloxone lasts?

Answer:  About 45 minutes.

Naloxone is commonly given intravenously in ICU for fastest action to reverse the opiod overdose. Naloxone act within a minute, and last up to 45 minutes.

If no IV access available, It can also be given via intramuscular or subcutaneous  route. Newer nasal forms are available too.

Objective of above question to emhasize the fact that, If patients do show a response they should remain under close monitoring as the effects of naloxone may wear off before those of the opioids and they may require repeat dosing at a later time.