Fitness Magazine ran an article last year about how much time you really need to spend on your workouts to maintain your weight. Harvard did a study to settle how much daily exercise you need to fend off weight gain. In the fine print of the study, after following over 30K women, it all boiled down to how many calories you burned rather than how much time you spent doing it.
So how that breaks down is if you do a tougher workout such as running at a 10 mile per hour pace-you would only need about 2 hours of exercise to burn enough calories (about 1,280 based on a 140lb woman), to maintain your weight. Compare that to if you take a 20 minute mile pace for walking, you would need about 7 weekly hours of exercise.