Hrm, I got mine on Saturday. I plugged it in and let it sit until the battery showed fully charged, after that I don't think I've plugged it in but I've used it for an hour or 2 each day. I have a 128mb SD card that I have all my books on, so I loaded from that.
My battery shows 80 - 90% full.
Okay I might have plugged it in since Saturday but not in the last couple of days.