I got my extended battery a few days ago. I had been curious exactly how long this thing can last. I know Nintendo claims "Up to" 8 hours, but does not state under what condition to make 8 hours.
I timed how long the battery lasted just for fun. This was hardly scientific or anything but I thought I would share my results.
I had the GamePad on with the main menu on the screen most of the time. I played a little Wii Fit U but that was about it. Most of the time the system was idle. I don't know how much playing an actual game would make a difference, but I wouldn't think it matters too much.
My battery lasted 6 hours and 32 minutes on Brightness 4.
Take away from this what you will!