Hmm, tough to say. On this machine, I have a 10gb, and it stays pretty full with general software. All of my video I keep on my xbox, in an 80gb, although I only use maybe 15 for video. I'm not much for keeping loads of mp3s. I do have a 60gb downstairs on another computer where I have a few hundred, although most of those are thanks to my brother, so that's maybe 3 gigs of mp3s.
Surprisingly, I dont use up a lot of space.
Recently Ive been thinking of creating a network databse with mp3s of all my cds, that will push my storage requirements up quite a bit.