Half a TB, and still I run out of space. Using HuffYUV and TooT efficiently sucks back free space like you wouldn't believe. So does keeping the results of failed experiments, and things you know are obsolete but haven't had time to go back and fix.
The next drive I buy (and it shouldn't be for a while, I hope) will be 200GB or better. Yikes. I remember my first IBM clone: a 286, which had a 20MB hard drive!
Really, two 80GB drives should be ample. But to paraphrase whoever it was (the Duchess of Windsor?), you can never be too rich or have too much drive space.