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.
You must be able to put the PFD on in a reasonable amount of time in an emergency.
Vessels 16 feet or longer must have one type of IV PFD.
throwable PFD
Must be in good, serviceable condition It is a legal requirement for a PFD to be in good, serviceable condition.PFDs must be properly sized for the intended wearer.
A PFD is a personal flotation device. It is important that it fits correctly and it must be approved for your weight range.
under the age of 13
16 feet
Children under 7 years of age while underway on the open deck of any vessel must always wear a PFD. A PFD is a Personal Floatation Device. The PFD must be an approved type such as type I,II, or III
16 feet
A type V (5) PFD is a Special use, or windsurfing PFD.
Every time the PFD is worn