I think this really depends on how healthy people are. My grandparents all lived into their 90s and only needed care for the last couple of years. My parents are almost 70, as are all their friends, and everybody is ridiculously healthy and able to live on their own. I think that overall health probably has a lot of do with how much care is eventually needed: it seems to me that when people are generally fit and eat health food, when they get do actually get near the end of their lifespans, they suffer very little. This is not just based on my own experience, but that of my parents and their friends who all worked in health care.
So perhaps the best advice for us CF people is to eat healthy, get lots of exercise and save a little bit for those last few years when we'll need in-home or nursing home care.