They last until they break.
That was precisely the problem with my gorgeous 27 inch Sony Trinitron XBR. One thermistor failed in the 25 years I owned it and it cost only $100 to repair as good as new. I simply got tired of waiting for it to break last year and finally bought a 55 inch Panny. As we near retirement age, I'm hoping to get good life out of our two flat screens as it will become harder to replace them on a fixed "income" (if there is any income at all in this interest rate environment).