That depends upon a few factors, such as what you mean by "in use" and the charging source.

For instance, my N800 drinks its fill of delicious electricity in well under eight hours each night while on, but idle, and charging via the AC adapter with which it came. If I leave the display set to stay on while charging, it does take longer to reach a full charge, but still well under eight hours. I'll venture a guess that it's closer to half that time.

On the other hand, I have also depleted my N800's battery while it was charging. I had the display on full brightness, I was streaming audio from an Internet radio station, and I had the speaker volume turned up to maximum. Additionally, I had some other applications open doing various things in the background, and I was only charging via a USB adapter and an AC-to-USB wall wart.