After doing some research by talking to a seat belt manufacturer, visiting the FIA & SFI websites, along with the NHTSA website and if these sources have their facts straight, I have learned:
1. OEM Porsche seat belts are polyester
2. FIA approved seat belts are polyester
3. Polyester is superior to Polyamide (Nylon) in strength retention over time and exposure to UV light, bacteria, water, acids and corrosives.
4. FIA recommends a 5 year replacement schedule for polyester belts used for motorsport they sanction.
5. SFI approves both Nylon and polyester belts and recommends a 2 year replacement cycle for either belt type, preferring not to differentiate between the materials and going with Nylon's limitations for motorsport.
6. The Federal Government does not have a replacement recommendation for factory belts used on the highways, probably because of the high cost per life saved of such a law.
7. If the Fed. Gov't. hypothetically did have a replacement schedule, the manufacturer I talked with said it probably would be around 10 years for a polyester belt.
8. Much of a harness' longevity depends on exposure to light, water, crash loads (crash with any belt bruising of body = replacement at any age), bacteria, solvents, oils, etc. as well as general wear and tear. So, over the same period time, an always clean, always dry, belt in a coupe' stored in a dark garage is going to be stronger than a dirty belt that is in a convertible driven a lot with the top down.
9. The load on a belt in a crash is dependent on vehicle speed and weight of person being retained. A 125 pound person would put half the load on a belt that a 250 pound person would. A 100 MPH crash (think open track event) involves the harness being exposed to 4 times the kinetic energy that a 50 MPH crash (think parking lot Autocross) does, so speed matters.
Given the above, I am off to replace my 2002 SFI, Nylon harness that I bought last time this issue came up, as well as my 1992 FIA Schroth factory polyester 6 points, with new FIA polyester replacements, but I think the guy next to me in tech with a 10-20 year old OEM belt perhaps should replace as well - if optimum safety is our goal. Just a thought