In the ultimate display of geekiness ...
Here is a plot of times people recorded in the Taupo Half Ironman in 2004 compared against the times they (assuming there aren't 2 seperate people having the same name each doing one of the races ...) recorded in the full Ironman 2.5 months later.
Basically, it looks like, for these 2 races, full ironman time is approximately 2.2 times the half ironman time, which puts me just over 10 hours. That's an AVERAGE of course.

