My friends give me guff all the time for having a buttload of music, almost half of which I don't even like. I like to think I'm prepared in the event someone says "Hey, do you have that song by ______?" "Why yes, yes I do."
Currently my iTunes is 41,624 songs, and about the only genre that hasn't been touched is christian rock (unless you count POD). It's constantly growing as I soundhound the college radio stations and hear about new bands from obscure websites. To be honest I don't think it'll ever stop growing.
So how big is yours?