How many calories in a pound of fat?

If I'm expending 500 calories at each workout, how many days would it take to actually lose a pound?  I know it's not an exact science, and some of it is water weight, yadda yadda, but just a guesstimate?

I need some motivation.  I've been sticking to a 1500 cal/day diet, and trying to get to the gym more often (2 days a week is all I can currently manage) and I'm really trying to get in more often.  If I have some sort of concrete measurement/goal, I think it might help.

1 lb. of fat is about 3,500 calories, however, it's not an exact science, and you're just as likely to lose muscle as fat, but...

To answer your question, 1 lb. of fat is 3,500 calories.

see this article for some info... http://www.hhp.ufl.edu/faculty/pbird/keepingf it/ARTICLE/fatcalories.HTM