Are you asking how much film length they can hold, an average 4.7Gb DVDR disc will hold approx 120mins of film. And a double layer 8.4Gb DVDR disc can hold approx 240mins of film. (Although please note that I have always found it holds a bit less than these figures. But you can always copy a film onto 2 discs with software such as DVD Fab.) If you are looking for cheap discs look at http://www.bigpockets.co.uk ......... Hope it helped.........
If you author the film with MP2 audio, you can easily get well over 2 hours of film on a 4.7GB disk. 'Stalker' fitted comfortably onto one at 165 mins. Also depends on the bitrates you set.
Useful site here:
http://www.videohelp.com/