10 months
For a long time I listened to my collection of music on a random album basis. iTunes was one of the few programs that actually lets you randomize by album instead of track. While trying to find a sample in a song that borrowed from a movie I decided to start listening to my music collection in alphabetical order by album (I never did find the sample I was looking for). This is all based off of listening to music on my computer when I'm working at it. I also listen to a lot of music at work and on CDs. Yes I still listen to CDs since I have a decent enough stereo system that CDs still sound better and don't require me to have cords running all over the place.
While looking for a piece of music today I looked over the last played times of all my music. Turns out 10 months later I've started my second loop. Meaning it's taken me almost 10 months to listen to everything in my music collection. iTunes reports that I have 21.6 days of music. Roughly that means that I'm listening to music on average 1.75 hours per day. That also means that I'm sitting at my computer that much per day. Scary thought.