A recent survey from Columbia (1) shows some interesting results. They asked 1600 participants between the ages of 18 and 64, how long they prefer to live. It appears that close to half of the surveyed population would not want to live more than the average life expectancy and almost 1/6 indicate a preference lower than that. Beyond the obvious correlation to expectations of the quality of life in old age, the data also shows significant relationship to race - with African Americans and Hispanics wanting to live a lot longer than White/Caucasians.
This is an important question and a rich area for further research. Till recently, humans had conquered most of the pathogens but they succumb to auto-immune diseases in which the body attacks itself. Heart disease, cancer and diabetic complications top the list - all of which portend lower quality of life in later years. This coupled with a brain that is bored could be a potent combination for humans, whose infrastructure was never designed to go beyond a few decades. In spite of the billions going down the drain to “cure all diseases,” it is clear that homo-sapiens may require outside help to get over their genetic deficiencies.
How long does one want to live? I think it should depend on whether one is able to add value to society.