Two recent studies emphasize an hour of exercise each day, five days a week. The first, from The Journal of the American Medical Association, studied over 34,000 women average age of 54 over 13 years. Women gained an average of 6 pounds. The only group that did not gain the weight was the normal weight group that averaged an hour of exercise five days a week.
(Did You Know? The exercising in these as well as most studies ranges from moderate walking to trainer-assisted programs.)
The second study from the University of Pittsburgh was published in the Archives of Internal Medicine. They followed nearly 200 overweight women ages 21-45 through a two year weight-loss program. They were calorie restricted and put through varies exercise routines. The women who lost weight (at least 10% of their starting weight) and kept it off exercised an average of 68 minutes a day, five days a week.
The conclusions we can take from these studies are that if you have some extra weight that a combination of calorie reduction and exercising at least an hour a day, five days a week will help reduce weight. If you are currently near your ideal weight that to maintain that you also need to exercise an hour a day, five days a week.