To keep a steady weight, a person needs to do enough physical activity in a day to equal about 30 minutes of moderate activity. This should maintain their weight.
It depends a lot on your lifestyle. Some people naturally have an active lifestyle. (Walking instead of always driving, for example.) But most people should have about 30 minutes to an hour of...
30 mins of exercise a day, everyday will suffice. Just make sure that your heart rate is at the optimal level at least about 25% below your maximum heart rate during that 30 mins.