High-intensity interval training (HIIT) involves performing repeated bouts of high and low intensity exercise. The length and duration of the high intensity work bout will depend on what you are trying to acheive.
For instance, a longer interval requires a greater amount of aerobic energy production. This occurs because longer intervals need to be slightly lower in intensity than shorter ones. For example, how long can you sprint for? If you give it your full attention, then probably 30 seconds.
Shorter, more intense intervals, such as sprinting, involve a greater amount of anaerobic energy production. This is the energy system that gives you a lot of ATP (or energy) in a short amount of time but it is also an energy pathway that fatigues quickly and produces lactic acid in your muscles. That's why higher intensities can only be sustained for a short amount of time.
In short, here is a menu for the least amount of exercise for the most amount of gains. It totals about four minutes per session, and you might want to try fitting this in to your schedule twice a week. Kenyan marathon runners (check marathon winner stats and you'll see that they are well populated by the Kenyans) use HIIT to boost their endurance.
1. Warm up: 3 minutes. (Light jog - basically a fast walk).
2. Sprint: 30 seconds, as fast as you possibly can. No kidding. No holds barred. Think "vomit-inducing."
3. Warm down - a light trot or even walking for 90 seconds.
Repeat steps 2 and 3 seven or eight times.
If I can't do this outdoors, I hijack two treadmills right next to each other. I have one on 10 or 12 mph and the other on 4-5 mph. I switch (carefully) between as I need to sprint or warm down between sprints, and I not only boost my Growth Hormone production (rejuvenation and injury repair) and my mitochondrial function (metabolism and effective cell turnover), but I also learn to be nimble as an organ grinder's monkey. Haven't you seen those treadmill fail gifs on the interwebs?
Be careful out there.