By exercising moderately for 30 minutes a day on most days, every day if possible, you can reduce your risk of heart attack, stroke, diabetes, high blood pressure and other medical problems.
If you are trying to lose weight, then exercise 60 to 90 minutes a day every day, if possible, can help you lose weight.
For detailed advise on exercising safely you can download Your Guide to Physical Activity and Your Heart (1) which is an outstanding 55 page book available from a division of the National Institutes of Health.
(1) Your Guide to Physical Activity and Your Heart from the National Heart, Lung, and Blood Institute. June 2006.