About an hour per GB.
It varies since the Data in the Video stream varies. A show that just has a static image, such as one of those Dish "You are watching satellite 119", would compress very efficiently so an hour of that would not take up very much room. Another channel with more movement would take up more for the same hour.
So it varies on content of your recording. But I think that most DVRs with a 250GB Hard Drive advertise themselves as 250 hours of SD recording a tenth of that for HD.