if the display is off, I'd imagine the mp3 player would last a long time - probably about 20% less than whatever the standby time is.
It is hard to say how long, because it could be 2hrs or it could be 24hrs. Depends on what widgets you have loaded, what apps are running, what your various polling frequencies are for email and whatever, etc ....
I was sitting on the beach for about 4 hours while listening to XM/Sirus and when I left I had 60% battery left. I started with a full charge. I also use while I run and go the gym and it seems like it does nothing to the battery for the hour or so. I also have beautiful widgets, battery left, executive assistance and calendar widget running.
Streaming hits the battery quite a bit harder than when you have the file on your phone.
I'm stickin with my sansa though. So small, so easy to pause/resume/ff & rewind, so cheap, such a long battery life, FM radio, voice recorder...I wish the droid had a sansa physically in it that was independent of the main device. That would be cool. Listening to stuff on my phone seems to run down my battery a bit too fast. Maybe I should try it out more.
Playing music has minimal drain on the battery. I play about 1.5 - 2 hours of audio on my Droid X playing via bluetooth on my daily commute. I last charged my phone 48 hours ago and battery is still at 50%. Apart from music, I have used the phone very lightly in this duration, i.e. about 10-20 minutes of calls, approx 1/2 hour of screen on time and no GPS, navigation, etc. Hope this gives you an idea.