So my zune lasted about 10 hours before it died on me.  I listened to two 45 minute long podcasts I'm subscribed to for the last hour or so of it, which I assume use more energy than just the normal mp3s.  I say that because the battery bar seemed like it had quite a bit more life before I started listening to the podcasts.

I'm pretty happy with that, and I ordered a car charger for it.