A high-capacity battery for the Wii U is now available in the United States, promising 8 hours of use with a single charge. The battery is available today through Nintendo's website for $32.
The standard Wii U GamePad battery lasts around 3-5 hours, though all battery life depends on a number of factors including usage of wireless communications and the brightness setting of the screen, among other elements.
You'll need a screwdriver to install the new high-capacity battery. The new high-capacity GamePad battery is already available in Japan and the United Kingdom.