I think it depends on how well it is coded. I have seen some really bad coding that makes me go "really what were you thinking?". But most of the ones I have seen just, the camera will just phone home to the server every once and a while and say "I'm here, keep the tunnel going". Once you actually hit it the website than the camera wakes up and starts sending a stream to you. If you have alarms set, are constantly recording to a remote server or it is a publically accessible camera etc, than you could be using quite a bit more bandwidth.
