What is the Least Amount of Exercise You Should Do?

What is the least amount of exercise you can do in order to obtain the maximum benefits. I have recently written several times about the apparent benefits of High Intensity Interval Training. http://notjustforboomers.blogspot.com/2013/04/high-intensity-interval-training.html. According to research cited in those posts, as little as three minutes or so a day for three days a week can have significant benefits for some individuals. Why then would anyone do any other type of exercise?

The answer to that question appears to be that because most of the studies on HIIT were relatively small and of brief duration, it is unknown how long those benefits will continue to accrue. Also, some of the studies supporting HIIT have found that it has little effect on the building of muscle and weight control.

In an article in today's New York Times, "The Rise of the Minimalist Workout, http://well.blogs.nytimes.com/2013/06/24/the-rise-of-the-minimalist-workout/?hp&pagewanted=print, the author points out some of these caveats. In my mind, the ability to conclude a workout in a matter of minutes rather than hours, far out ways any questions about HIIT.  I intend to keep to my program of three twenty second intervals on an exercise bike at full speed just three times a week.  I feel better and it saves an enormous amount of time. What do you think?