The software does not use a single life expectancy / mortality / age of death assumption.
Instead, it does a much more sophisticated analysis in which the present value of each year’s payment is calculated. You can see that calculation if you scroll to the bottom of the “more info” screen for the pension, and click the link for the audit trail.
If you wanted to know the life expectancy for your client, you could simply Google something like “life expectancy United States male.”