Just this very minute, I finished listening to all 5,609 songs on my iTunes, which is 21.34 gigabytes worth of music.
I began the process on October 10th, so apparently it takes me about four months to cycle through my entire collection.
If I listened to it nonstop, iTunes claims I could get through it in just 15.3 days. I assume that would mean listening for 24 hours a day.
Anyway, it's time to go home from work now, and tomorrow I shall begin the process anew.