Microsoft's Cesar Menendez publishes battery life info for the Zune 2 devices on the Zune Insider blog:

Ok, we've finished testing the battery life, and this is what we found:

Zune 4 and Zune 8
Audio: up to 24 hours
Video: up to 4 hours

Zune 80
Audio: up to 30 hours
Video: up to 4 hours

We did all testing using pre-production hardware and software, with the wireless turned off. Audio was tested using 128kbps .wma tracks, and video was tested using 500kbps .wmv videos.

So how does this compare to the iPod? According to Apple, the iPod nano (which competes with the Zune 4 and 8) gets 24 hours on audio and 5 on video, while the 80 GB iPod classic (comparable to the Zune 80) gets 30 hours on audio and 5 on video.So these are very comparable, though the iPods seem to get a bit better battery life on video for some reason.