The post comes with this chart. For each country and each gender, he compares the life expectancy and the average retirment age, the difference between which yielding the "number of years in retirement".
Unfortunately, one should be careful when taking the difference between two averages. This is because the population that underlies one average may not be the same one that underlies the other average.
The life expectancy is clearly an average over the entire population. Average retirement age, however, is almost certainly an average over people who live to at least retirement age. Thus, taking their difference doesn't make sense. What we need is the life expectancy of those people who at least live to see retirement.
The (somewhat) funny result of taking the difference of those averages can be seen in the case of Mexican men (the bottom blue bar). By this metric, the average years in retirement for Mexican males is essentially zero: they work till they die.
Another way to see that this metric can't work: if the average years in retirement is zero, then some will die after they retire while others will retire after they die! Retiring after death is quite literally captured when you substract the average retirement age from the average life expectancy!
Take all the age groups above the retirement age, compute weights for each age group as a proportion of all remaining age groups, use these weights to compute the weighted average remaining life.
For Mexican males, this yields about 7 years, assuming the retirement age is indeed 75 as shown on the chart above.
P.S. See also the comments from the Flowing Data readers.