You said, "Does anyone know the expected life of a tent at Everest base Camp? I suspect it's in the weeks or a month or 2 before it is damaged beyond use by UV."
The average nylon fly’s useful life at sea level, during full sun days, is 21. There are two base camps on Everest and the higher one is 5,545 meters (18,192 ft.). With every 1000 meters increase in altitude, UV levels increase by 10% to 12%. So, 5,545/1000 * 11% * 21days = 12.8 days, rounded to 13. This passes the real world experience test as is attested to by Bob Gross in his post at http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=38299 assuming his tent had some use prior to leaving it on a summit for nine days.
He said, “While doing a mountain expedition, my team left one of my large dome tents erected at base camp. We came back after it about nine days later, and it was "brittle" from the sun exposure. I won't state that the fabric was totally ruined, but I never wanted to use it again.”
If the fly was polyester it would last approximately 61% longer in each of the above scenarios.
The above calculations are worst case scenarios and the time could be lengthened by the following known variables, plus possibly others: sun height, latitude, cloud cover, altitude, ozone, ground reflection, and fly color.