OK the ladies are right, it will come off your BMR. My BMR is something crazy like 2100 cals!! (I'm extremely overweight obviously). I eat to 1200 cals and exercise 500 cals a day and I tend to drop around 2kgs a week.

As for the exercise bike, it really does depend on your weight etc, but fr me it generally takes an hour to burn 500 cals (and it's easier for me to burn cals than people lighter than me because I'm so big and it doesn't take much for my HR to go up and stay up).
The cal counters on gym equipment are notoriously inaccurate. :S
At my size, I burnt 1200 calories today in 1hr 45 mins.... with an avg HR of 167.