I once asked a salesperson how much video an 80GB hard drive holds and he answered "80GB worth" -- meaning it depends on how much you compress the video.
I'm using Handbrake (on the Mac) to compress DVDs to MP4 to playback on the iPod and with the settings I am using (decent quality at smallest file size) a 2hr movie takes up around 600MB of space.
Hope this helps.
12" iBook G4 Mac OS X (10.4.3) 30GB iPod with Video