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.
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
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
Inflatable PFD
Every time it is worn!
a pfd for everyone aboard
A PFD for everyone aboard
a PFD for everyone aboard
a pfd for everyone aboard
Wearing an inflatable PFD while on a personal watercraft.
Life Jacket or PFD for everyone on board