Turns out my memory was pretty good. I just went through and calculated my 2 TB drive, with all mpeg-4 HD recordings:
73 hours recorded, 18% used
73/(0.18) = 405 hours total capacity for 2 TB
2048 gB / 405 hours = 5 gB per hour
All of this will vary by program material, of course, but should get us in the ballpark. I've done this calculation on multiple drives and multiple D* boxes and pretty much consistently gotten between 400 and 450 hours per 2 TB drive.
Perfect, that is close enough for what I was trying to figure out!