I've heard many of us here say when we're ready to be done, we'll be done. That seems at odds with much of the rest of the country.
What brought this up is my brother best friend, we've all been friends since high school, and we've had some great times, we are 56 and 57 respectively. This friend has Type One diabetes, his health is truly abysmal, loss of vision, multiple skin cancers, kidney transplant, heart issues, and the kidney transplant is now failing. He is desperately searching for a new kidney donor (living not cadaver). This amazes me. First that doctors would think this is a good idea, but also why he is so set on going on. His life is misery personified (his own claim), so why continue it, it baffles me. I guess not everyone is okay with the idea of dying, I know my mom was terrified of it, she couldn't understand why I wasn't.
So, my confusion aside: Long enough for me it's as long as I have the capacity and desire for love and happiness. Many things can grind that away, but it's my personal benchmark.