Apparently a lot!! Check out this article I found on Yahoo! that notes how long you'd have to work out to burn off popular fast food items. If you want a simple formula to follow, try this:
The average person burns about 100 calories per mile run. If I run a 10 minute mile, then it would take me 20 minutes to burn off a Krispy Kreme glazed donut (200 calories).
I haven't allowed this kind of stuff to guilt myself out of foodie pleasures, but it does make me rethink choices like soda and other junk. I mean, I do run so I can eat!