Depends really. PLaying music/video will certainly result in loss of power earlier than reading a PDF.

With light web browsing, RSS feed reading, Mauku checking/posting, e-mail reading and writing, my tablet goes all day and then some.

The issue with using it as a media player (music) is that you'll crank up the CPU and increase memory usage, both of which will result in lower battery time.
Streaming music will also hit the battery since wifi will be cranked.

Don't expect to have iPod classic type of battery life with this thing.