I burned an extra 10000 calories from cycling last week and didn't lose a pound (and wasn't trying to, btw). Which means, of course, that unless you're keeping track of how much you burn and how much you eat it's hard to diet sensibly.

I would recommend no more than a 1000 calorie per day deficit, preferrably only 500 (i.e. 1 pound per week). It's way better for you in the long run to lose weight slowly.

--Steve