I rarely turn mine off (it's usually turned off by accident). I turn on whispernet in the morning just long enough to download my newspaper subscription. I can easily go a week without recharging and closer to 2 weeks is not that unusual. If I feel I'm going to be using whispernet for along time, I'll plug into power while I surf.
I've only let my battery run down low enough to get a warning message once as an experiment. I wanted to see how well and fast the Zire USB cable would charge the Kindle (thanks TallMomof2). It works nearly as fast as the AC charger.