Disclaimer: I work in a hospital setting which sees an awful lot of elderly people. Before I started working there I would've said 'as long as possible'
Two years down the line...not so much.
I'm sure we all know someone who's 101 and living a fabulous life, but for the majority, 85 and upwards is no bloody fun at all. A slow descent into falling apart, increasing hospital admissions, more and more meds, falls, dementia and then death in some hospital bed because you fell over and hit your head and nobody noticed you were more confused than normal.
I'm going for 75-80. After that I think i'd be happy to shuffle off this mortal coil