I've had a good number of tubes test as worn out immediately after receipt from some Ebay purchases ("worn out" meaning testing below minimum GM or emission levels). Can't say that I've ever worn out a pair of new-testing tubes from use, although I've had some that have developed noise or significant microphonics after a period of time. As far as life span on a 6SN7 (or any other tube for that matter), a lot depends on how it is run in the circuit. If it's run at conservative voltages, then a 6SN7 can probably last 5k hours, maybe more. If it's run at the upper end (or max) of its ratings, then 1k hours may be the max. I've seen stories of Western Electric 300B tubes still going strong after 20+ years of 24/7 operation in telephone equipment, although I would imagine they were run quite conservatively in that equipment. I have 9k hours (and counting) on the KR 300BXLS output tubes in my Jota amp. While they don't test at NOS levels, they are still well above minimum. My tester has settings for a regular 300B but it doesn't have specific settings for the higher-output XLS version (which were developed long after Hickok disappeared from the market), so I can only get an approximation on my tester. But I can't tell any sonic difference (yet) when swapping in a new pair that I have as backups, which is a more valid test.