It's vastly dependent on the format. You have 3 standards and then varieties of each. Most clips are mpg (mpeg), avi, or wmv. A dvd is generally in vob.

Mpg is generaly 100mb per 10 minutes of sound and video. Avi with xvid or divx 5.2 encoding and mp3 sound runs about the same. So a standard 2 hour dvd is gonna run about 1.2 gb.

Wmv which is used by novice encoders and ghetto websites is compressed much more and ends up being smaller.

Vob the native dvd format (mpeg 4) is huge, figure on 4+ gb for a dvd.

Resolution is also a factor, a bigger image naturally takes up more but then it looks better on a 46 inch screen.
For ease of math say 2hours = 1gb, so 2000gb x 2 hours = 4000ish hours.

Someone mentioned music earlier, I've been working at ripping my collection onto my machine. At the moment I have 80.5 gb which is approx. 1400 albums. Figuring the average length of an album at an hour, it would take me 8.3 weeks to listen to them all going 24 hours a day.

I recently was going through all my techno/electronic/trance stuff to thin it out because there are so many shitty musicians and at 6-8 hours a day it took me almost 3 months to listen to it all.