I expect Ginsburg may last a lot longer.
My mother turned 92 last month, and is failing both physically and mentally...but it has been gradual.
My MIL turns 90 in May, and she remains very alert mentally. Physical not so good.
My point is don't get hopes up about Ginsberg. If Obama and the entire democrat establishment couldn't persuade her to step down, I doubt she will leave the court, short of death.
I am not aware of a provision concerning mental failure, of sitting justices--is there?