The full details of the Wii U's GamePad battery life have been revealed, and they're concerning.
IGN has confirmed while using a retail unit that not only does the GamePad last for a measly 3-5 hours, but recharge time is a staggering 2.5 hours. As a result, the GamePad docking cradle, which is included with the Deluxe Edition, will prove invaluable. Outside of that option you're stuck either charging it frequently or leaving it plugged-in, a solution that is hampered by the relatively short cable that requries a connection to a wall outlet.
If you find this bothersome you can opt to use the Wii U Pro Controller that has been reconfirmed to last a much more generous 80 hours with a 4.5 hour recharge time. Of course, some games will require the GamePad leaving you vulnerable to the GamePad's short battery life.