I just saw the other post of yours, so are you trying to lose 1 ponnd of body fat? You could always try the Jean Val Jean method! I think you lose 1 pound for each loaf of bread you steal, or something.
Well, when I go to the gym, I do 15 kilometres on the exercise bike. It takes 35 mins and the machine tells me I have burnt approx 250 calories. So thats about 240 kilometres on an exercise bike. Of course you wouldn't have to do so much on a road bike - I'd estimate 180 - 200 kilometres.
On...and I should add that these figures can only be very approximate. Everybody burns calories at different speeds and there is no way that the exercise bike can judge variables like my weight and the rate that I burn calories etc.