You would probably lose something in the order of 600-700 an hour if you keep a decent speed up.
Saying that, you say speed (technically resistance_ on 5... out of how many? At the gym here at work they only go up to about 12, but at the one I belong to, they go up to about 25.
5 on the work gym bike feels very different to 5 on the other one!
Try and maintain a speed of about 80rpm.
Good luck.