How much data is needed to calculate a representative FPT?

One month? 6 weeks? 6 months?..

Depends on how many max efforts exist in your data. Often one good challenging variable ride will do. Rarely someone will ride for months without ever reaching their limit.

i’m in this category… i’m confused if i should just keep what I had from months ago or not, Or do I need to disrupt my training plan with a major hard testing workout?