With the N800-provided 890mAh charger it takes about 2 hours or a bit more to charge from low level to full (I prefer to charge it way before it drops that low but a couple of months back I had some situations where it accidentally discharged overnight due to rogue applications). With some of the smaller Nokia phone-chargers (390mAh I think) it takes an hour or two more.

@trido:
It sounds like something happened with the unit's charging control. And yes, you can charge it while it's off, and while it's on, and you can use it while charging (although when running battery-intensive applications or streaming internet radio you'll see it charging for a very long time).

Try stopping the charging. Use it for a while then try again. If that doesn't fix it then it sounds like something happened to the device and it may need a repair. It could be OS2008 though, but as there aren't other reports.. well, it could be worth trying a reflash.