It depends on what you use it for.
For me, I have the following space usage:
Laptop- 30GB SSD. School use laptop, with Word/Ppt/Excel/OneNote/Outlook 2007, Photoshop CS4, Google Chrome, Trillian, GTalk, and Audiosurf; they all take up about 25 GBs total. I have literally hundreds of documents and such taking up another 2GBs, as each document is only a few kilobytes in size.
Desktop- 1TB HDD. Gaming desktop. Has all my games, programs, music, and videos. Too much to list. Most of my space is used up by my programs, as each game is ~5-15GBs.
Externals- 2TB HDD. Storage external, contains all my program disk images, music, photos, videos, movies, tv shows, and backups of all my documents. Most of the space is used up by my movies and programs.
Hope that helps you out a bit. For me, a good general rule of thumb is:
School use = 40GBs