Audio files assuming 10:1 (128kbps in mp3 ) compression approx 10 cds on one cdr (650Mb) , with win media player 20:1 for the same audible quality 20 cds on one disc.
Video files on a dvd recorder at 'standard' setting, 2hours of video on one single layer dvd (4.7Gb) ,you can get loads more if you tweak the compression on the recorder (assuming you are using a dvd recorder not a pc to do this) Best you can getaway with IMO is 3 hours per dvd but is dependent on picture content . I use this for VHS transfers and doesn't look too bad but too much movement or texture (grainy / noisy images ) will result in juddery and blocky pictures. You can double this for a dual layer dvd which is 9.4Gb.
You can of course use a pc to do the same but there are many more variables including avi files or mp4 / divx which can cram even more onto a dvd.