You've raised an interesting question ...
... and one that has been discussed here before. There is no way to know how long an A has to live unless a medical doctor makes a diagnosis. And, even then, I believe there are A's out there with nine lives who can beat the odds.
My AH is what I call a "dead man walking." He is a shrunken, skeletal being. Absolutely NO skin on his behind whatsoever. He wears 30-inch waist jeans and they're falling off him. He has a scruffy beard, no longer wears contacts, (prefering to wear an old pair of wire-rimmed glasses), has what appears to be neurological problems - facial tick, shaking hands, staggering gait. His memory is shot and a lot of his everyday cognitive ability is gone too. Commonsense things baffle him and he remembers almost nothing, he even has to search for words to link a sentence together most of the time.
How long will he live? Don't know. He never goes to doctors, so heaven only knows what his liver enzyme levels must be. Regardless, he still survives. He has a very strong gene pool, so he could very well outlast me!
Regardless of how long he "lives" doesn't matter because he isn't really living. He exists. No friends. No interaction other than that he has with coworkers. No family ties. No interest in much of anything, other than drinking.
So I think when we ask how long they'll live, we're basically inquiring into how long an A's physiological functions will continue. What they're doing isn't remotely related to living.