Whether you are using the injectable naloxone or the nasal spray, they work within a couple of minutes in an opioid overdose. Some opioid medications are very strong and it will take two doses of naloxone to reverse an overdose.
If the person who overdosed hasn't improved by 2 minutes after administering Naloxone, give a second dose. Hopefully by this time you should also be on the phone with 911 for further instructions.
We highly recommend getting training to administer Naloxone here. It's online and it's free.
You can read more about Narcan nasal spray here