Seriously, it's one of my main gripe with the Wii U.
It seems to me that it lasts only for 3-4 hours when fully charged. But the worst is that if you don't use it during a few days, the battery is totally empty.... I never have any problem with my other batteries..... The 3DS battery doesn't go empty alone when not used, the PC batteries don't discharge themselves.... Only the Wii U gamepad battery does it.
How many time last your gamepad battery? How many of you are using a third party bigger battery? It seems Nintendo has chose the worst available battery.