Comes in 80 and 160 GB. 80 GB rated up to 30 hours for audio; the CNET labs got 45 hours out of it. 160GB rated up to 40 hours for audio, but CNET didn't test that version.

http://reviews.cnet.com/portable-video-players-pvps/apple-ipod-classic-80gb/4505-6499_7-32595955.html?tag=mncol;lst

Apple's standard for measurement is 128 kbps AAC. I would imagine that the same bitrate mp3 would produce similar results for battery life. Keep in mind that the higher the bitrate, the greater the strain for battery life as it will take fewer songs/minutes to fill the cache and the hard drive will need to be accessed more frequently. Battery life is also maximized by use of playlists. Skipping from track to track and doing a lot of search also reduces battery run time.

I could also suggested the Microsoft Zune 80, which is rated for up to 30 hours of audio playback with wifi turned off. However, CNET's labs got only 22 hours. But that is still within your range.

You didn't mention which players you looked at. There aren't a whole lot of current hard drive players out there. The only other one that comes to mind is the Archos 604 or 605. So, you may have already considered and rejected all of these.

Another option is to get a flash based player with 32GB Creative Zen, which has battery life up to 30 hours for audio. You can also add capacity with an SD card, since it has a slot for that. You could also go with a smaller capacity Creative Zen (comes in 2,4,8, and 16GB as well). And use SD cards to swap out music.

Good Luck!