World's Fastest Workout: 4 Minutes.
Adam Rifkin stashed this in Fitspo
Why can't you just quit after 3 minutes, 50 seconds?
High-Intensity Interval Training (HIIT): http://en.wikipedia.org/wiki/High-intensity_interval_training
Tabata-style workouts like this are efficient, but in my experience they are VERY easy to injure yourself doing. So even though the maximum effort part of the workout is only 4 minutes, you need to warm up properly beforehand.
Also you'll need a timer. I got one that's an iPhone app called Timers Pro.
done at a moderate intensity, this exercise is a great warm up too.