Tube Life Question: 12AX7 Drivers

Hello.

OK, here's a question for some of those out there who have lived with 12AX7 drivers. How long have they lasted in your experience? I used to have an amp that used them, and after 6 years, they still seemed fine. Now that I again own an amp that uses these tubes in this application, I'm wondering what the actual life expectancy is - 5 years, 10 years, 25 years???

Thank you in advance,
Joe