This is what Nate said, judge for yourself.
As a matter of actuarial science, Francis is not likely to have an especially long tenure. The nine previous men who were known to be 75 or older at the time of their election to the papacy served for an average of only five years.source
That may be too pessimistic an estimate in the case of Francis: it is based on a small sample size and does not account for modern improvements in health and medical science. Acknowledging that there are considerable lifestyle differences between an Argentinian pope and an average American citizen, the life expectancy for a 76-year-old American man is now about 10 additional years. Of course, Francis could follow Benedict XVI’s precedent by resigning the papacy before his death. It is probably safe to say that a tenure shorter than five years would be considered brief for Francis, while a regime of much more than 10 years might be unexpectedly long.