I have to agree-it's soooo hard to guess. If it makes ya feel better, I would do the same thing as you......and NOT amputate. If he was younger and the vet could be sure that it would extend his life, then I'd go for it. I don't know, though.....if your vet honestly thinks that it'll give your cat more time and your cat has no other health probs, then maybe you can consider it. He would do fine with 3 legs. I'd call the vet and ask what he thinks.
My heart goes out to you and I wish you and your kitty the very very best. Spoil him rotten!!!! =D
That's not really an easy question to answer. I would guess six months, but it's largely a guess. A number of factors could change this, though.
A cat can live quite happily with one leg gone missing, btw. Should you chose to go that route.