According to the manufacturers instructions.
We do not know what you mean by "PFD", please do not use abbreviations. But the answer would be when the instructions for the "PFD" say it needs to be replaced.
replace it
You should replace the CO2 cylinder on your inflatable PFD (personal flotation device) after each use, especially if it has been activated. Additionally, regularly inspect the cylinder for signs of corrosion, damage, or a low pressure indicator, and replace it if any issues are found. It's also advisable to replace the cylinder according to the manufacturer’s guidelines, typically every 2-3 years, even if it hasn't been used. Always ensure you have a fully charged and functional CO2 cylinder before using the PFD.
Discard and replace it- do not attempt to fix it
Discard and replace it - do not attempt to fix it
For the newer Inflatable PFDs, most manufactures will tell you when you should replace your cylinder, and most have an easily viewable inflator status. For instance - The Mustang Deluxe Automatic Inflatable PFD: Universal fit states: No inflator maintenance is require for 5 years unless the PFD has been inflated. You can easily view the inflator status and replacement date through the safety inspection window. Green indicates ready for use. However, each manufacturer can have different specification, so it's best to check with your manufacturer and for your specific vest type. For more information regarding the Inflatables, visit: The link below
discard because they are unusable
Every time the PFD is worn
When selecting a PFD, the most important consideration should be size.
Every time the PFD is worn
Discard and replace it - do not attempt to fix it