I'd say very few people DO want to engage in HIIT training. It can't be called fun unless you have a good masochistic streak somewhere. Just getting to the point where you can judge your maximum 5 minute effort is very difficult and doesn't happen over night. It's easy to know you pushed too hard if you die before 5min is up but judging the flip side, simply knowing when you've pushing yourself to the max that you can do in 5min (or whatever your set time is) is harder to judge and it's physically really hard. I typically have to rest as long as I push if I want to have any chance of doing several sets.
http://www.sciencedaily.com/releases...0311123639.htm has a good article and it relates to me because I'm over 60.
I see HIIT routines of anything from 1 to 5 minutes. I'm not sure it matters. The underlying human physiology they both exploit in order to gain fitness is the need to stress the body and initiate it's response which is to strengthen your stressed muscles, including your heart. That's essentially the definition of improving ones health as well as cycling ability.
But out of all the cyclists in the world, not many are "cyclists" working at gaining that level of fitness. It's just important foundational information that "cyclists" should be aware of, at least conceptually, regardless of whether the choose to engage in HIIT.
Just as importantly, it should be recognized that you can, to some degree, choose your level of "hi". There's no magic number that I'm aware of but anyone looking to try to avail themselves of what we know now about gaining fitness, that your improvement in fitness for a given exercise correlates with the level of effort. Short hard bursts are better than long sustained efforts; gains in fitness are proportional (to some degree) to direct effort, not length of time in the saddle.
One might say that just about everything you do outside of the 20 or 30 most strenuous minutes of your ride , is pretty much just fun time for you. Put it in or don't put it in but you're strength and fitness are dictated by the level of stress you put your body under for those high effort minutes. Longer is fine...burns calories, is fun and one learns from simply having more time in the seat etc etc, but in terms of fitness, it's the max effort that's the golden goose.
I find that to be very interesting and very important - especially for the people that ride for fitness and that's not a small part of the over 60 crew.