An interesting article at the New York Times explores how much exercise one needs to do to improve her mood. If you are not active at all, as little as 20 minutes a week (and no, that's not a typo) can work, but the sweet spot is 150 minutes of moderate activity a week.
In case the link above did not work, here it is spelled out:
Have you discovered that working out regularly affects your mood? Is there a minimum you need to do to get that positive effect?