I currently do a 1:2 ratio of high to low intensity. My high intensity interval lasts 60 seconds, and my low intensity one lasts 120 seconds. I do this on either the elliptical or the heavy bag (I'm forced to shorten the intervals on the heavy bag due to the large amount of energy exerted).
I know that over time, you should increase the ratio until you hit 2:1 high to low, which is generally agreed to be the sweet spot for HIIT.
The problem I'm having is figuring out how long the intervals should be. I've seen suggestions as low as 20:10 seconds (known as Tabata) or as high as 120:60 seconds (intervals higher than that don't appear to be considered HIIT). Is the length of the interval simply not as important as the ratio?