My puny music collection stands at 9.97GB, 1691 songs, 98.5 hours, so about 4 days. However, according to some Last.fm statistic messing, I listen to about 2000 tracks a month. So, allowing for a good few repeats, once a month I can cycle through my entire music collection.
If you check out my Last.fm account, you can see my listening tends to focus on about ten bands. However, over the past few months, I've changed that up a bit by listening again to a lot of albums I had rarely listened to, or new releases from bands. I've listened to most of my library at least twice. By that point I decide whether or not I'll bother to keep the album. (If I do decide to keep it, I tend to go out and buy it). I don't tend to get entire discographies unless I really, really like the band, or they have a very solid (or very short!) discography.
Over the past eight months or so I haven't listened to music on the move very often, mostly because I haven't been going anywhere. At college, I always listened to my MP3 player on the bus. That was a 1GB model, so I had to do a lot of regular swapping of tracks. I now have a 2GB Walkman phone, and I'm trying to decide if I should bother to upgrade the capacity. Mostly I want to fit about the same number of songs (500) at a higher bitrate.