The extremes? In 2004, the average finish time was 12 hours 6 minutes; in 2013, the average was a full hour faster at 11:06. Same course, same world class athletes, roughly the same number of athletes.
How variable can finish times be from year to year? There's no real standard pattern; we don't see faster finish times each year. So what's the main difference? Kona's a blast furnace experience like no other triathlon in the world. It can crush you, or, in some years, the winds will be somewhat merciful; but it's never easy.
We've run the numbers for All Ironman Kona World Championships since 2002, and further segmented the annual average times by Age Group for each year. Here are the results of our analysis.
For more, see our complete Ironman Kona Coverage, Qualifying for Kona, Kona Qualifying Race Schedule 2013/2014, our analysis of Top 30 Toughest/Easiest Ironman Events, and the site's archive of more than 200 Stats/Results/Analysis posts.