Obviously it's about quality over quantity but I'd thought this would be an interesting topic anyways. Also, use as many measures of library size as you can readily get your hands on because using only measure could be deceiving (for example if one were really into opera, he/she wouldn't have many tracks but would still have alot of music in terms of play time.)
My Library: 17116 songs, 48.9days play time, 95.83GB (itunes)
Good to see so many responses in such a short period of time. I just want to encourage everyone to contribute, even if you don't have the biggest library. It's not a competition, just data for those who are interested.