Here's what Glen Raven has to say about loss of UV resistance:

"Thank you for contacting Sunbrella. As your fabric gets older, the elements do break down some of the UV protectant; that is why we recommend re-treating the fabric with 303 Fabric Guard when the application is cleaned. www.303products.com This is chemically the closest to what is put on the fabric during manufacturing. This will help put back repellency and UV protection."

Was hoping for (but not really expecting) something about X% loss in Y years, but that would be very location and exposure dependent. Note that Glen Raven says the 303 treatment only 'helps put back' the original resistance.
Guess I'll add a new sail cover to my list. When I got my new main, I had to add a panel to help cover the increased roach but really didn't make the addition big enough.