I have searched the DuPont Nomex site and they do not put a life span on the material - rather they state it's properties and alike, however I believe the manufacture of the Nomex PPE would be able to act as a guide - depends how often it is worn and for what.
Our ex Captains gear was 5 years old when he replaced it - not due to excessive operational use (i.e. at the sharp or pointy end), fading, tears, cuts or alike, but because it had been washed so many times that it started to fray!
Maybe try FARA - they import it and make PPE from it - they should have an answer -but then how long is a piece of string?