PFMEA is gradually built up during development (initial/draft versions), and matured towards the completion of design transfer. Afterwords it is managed and updated as the product evolves and customer feedback is obtained.
I don't know to say that there is an "ideal" time to do it, but generally speaking - after building initial units (doesn't matter how many) is a good time to work on it.
Some small businesses start with
DFMEA and PFMEA together and later on separate them when the PFMEA begins to get volume of it own.
Shimon