Glenn Wolsey published an interesting interview with the owner of the largest iTunes music collection. The owner of that collection, Will Friedwald, maintains a massive archive of 849 GB of music equating to 172,150 tracks. It would take roughly 2.2 years if Mr. Friedwald played each song back to back. As you'll discover in the interview, having iTunes manage all those songs makes it rather sluggish.
I am happy to report that my 9GB of music leaves iTunes running like a champ. That 9GB equates to roughly 1,800 tracks, most of which were ripped from CDs I own, and 745 purchased from the iTunes Store. A few years ago I did have a much larger collection of music but I sat down one day and deleted songs and albums that weren't up to my tastes anymore. How large is your music collection?
On a related note, I am eagerly awaiting the release of Bandwagon DIY. I covered the launch of Bandwagon, an OS X application that backs up your iTunes media online, back in February. Since then, the Bandwagon team discovered that they cannot handle the insane amount of storage that people need for their media collections. As such, the upcoming Bandwagon DIY and DIY+ services allow you to backup your iTunes music to your Amazon S3 account, FTP server or Omnidrive. The DIY+ version adds the ability to sync music between 2 different computers.