It turns out that while in the aggregate we actually are pretty good at estimating life expectancy (i.e., ‘the wisdom of the crowds’ holds up quite well), at the individual level many people are drastically far off in their estimates, where some with health issues who predicted a ‘0% probability’ of surviving to age 75, and others with great health and thought they had a ‘100%’ chance really only have 80% odds.
Of course, the reality is that health conditions do vary, and as a result, the planned period of retirement should be personalized – both for individual preferences, and simply factual contextual data points like current health, smoking status, and even income (which is correlated to longevity) – which can easily result in one person having a life expectancy estimate that is 15+ years different from another (and in turn could change required savings to retire by 30%+!).
In practice, in most of financial planning tools, financial planners use a generic number such as 90 or 95 as someone's life expectancy,
Here is the good news - there are online tools available for you to use. The most popular option appears to be the site LivingTo100, which draws on data from the New England Centenarian Study to try to get a good picture of how likely it is for the client to live to age 100 and what his/her individualized life expectancy would be.