I am the one from the list of folks who do regualr workouts to remain healthy and strong. Not only this my daily routine is also very tough so if I miss my workouts then it will cover in my routine.
Daily half an hour of workout is enough no matter of what type any from yoga to aerobics or from heavy bodybuilding to normal exercise keeps you always healthy coz whatever is your workout the main aim is to improve and keep your body circulation system in needed motion.
Sure I do morning exercises every day and to my opinion it's the only way to be fit and healthy. So I guess that fifteen minutes a day is quite enough for that.