How many gigs to an hour?

I know that it depends on a lot of things but ... I'm going to ask anyway ;) Roughly, and I do realize that this is very rough, but how many GB per hour are in a typical streaming show? What I'm trying to figure out is let's say you watch streaming TV for 1 hour in HD, roughly how many GBs would that be? Then multiply that by number of hours I watch and x 30 days and I could have a rough idea of where I come to the data cap.