What's the ideal lifespan? TV entertainer Cilla Black told a magazine that 75 would suit her. After watching her mother survive to 80 while suffering health problems and having to be fed through a tube, the 67-year-old has revised her opinion about how long she wants to carry on living.

According to the Bible, we can expect to reach 70 ("threescore years and 10"), or 80 if we're strong, though when those words were first written, the average lifespan would have been much shorter.

Many of us are now living beyond 75, but there's no guarantee, of course, that our final years will be spent independently, in sound mind, and without discomfort.

It's something of a taboo to speak as honestly as Cilla Black about when one would like to die, but it's a question that many of us think about. When do you think is a good time to go?