You know my initial response to this question is, “as often as possible”, or “whenever you want”. But there’s more to my question than that. 
Last night I had the privilege of attending a series of presentations by social entrepreneurs from all across Toronto at the Centre for Social Innovation. These women (mostly) and men were focused on how we build healthier communities, and the solutions were as diverse as the presenters. At the event, I was discussing a rule-of-thumb I learned from a Parisienne acquaintance of mine: it’s the one kilometre rule. It works like this: if your errand or appointment is one kilometre or less from where you are, walk. No cab, no bike, no streetcar or metro. Just use your feet.
Sure, you say, that sounds great, but how do I know if it’s a kilometre? Won’t it take long? I’m a busy person! At a brisk pace, but by no means race-walking, a kilometre will take you about ten minutes. So if you walk to the store and back, you’ve gained twenty minutes of exercise that you wouldn’t have done otherwise. How long did you spend waiting for your last public transit, or looking for a parking spot?
It’s worth it for your health and a longer lifespan – instead standing around checking your smartphone, or circling around looking for a parking spot, invest that ten minutes in yourself. Your stronger heart and your clearer head will thank you.