Stephen Holt takes a look at 10 minute workouts:

Last week I helped Men’s Health magazine on a piece about getting workouts done fast. Personally, I think you can get a great body-changing workout done in minutes.
How fast?
I’m talking 10-minutes.
By now, I’m sure you’re asking…
How do you lose fat in only ten minutes?
or
What about the rule that it takes 20 minutes of cardio before you start burning fat?
Well, first of all, as I have said before, the whole idea of a fat-burning zone or a magical fat-burning time period just doesn’t fit with the common sense approach to fat loss.
People can lose fat without doing any cardio at all - so its clear that you don’t have to workout for at least 20-minutes…

In college during my busiest year, my junior year, I adopted this principle of a 15 minute workout. It comprised of nothing but pushups and situps. No cardio, no big time, just every night before I showered for bed. It was my best regimen so far, and I still think about adding it back into my mix to see if it is still as effective.