The current recommendations for exercise rest around 150 minutes per week. Yet researchers in Taiwan observing over a period of 8 years noticed that even at 15 minutes per day, deaths from any cause were dropped significantly. It may not seem like it, but you can do a lot in 15 minutes.
15 minutes is enough for a tough circuit or a brisk walk around the block. 15 minutes in plenty of time to get 4 or 5 hard sets of just one exercise in, or even to simply take some time to stretch out your stiff muslces. I think it's safe to say that most people, including myself, wish we had more time to work out and take care of ourselves. Use this info as a reminder that "something" is, in fact, better than "nothing".