The conspicuously 360-like Wii U Pro Controller should last you well into your gaming sessions, according to an estimate found on retail sites. The figure was probably provided by Nintendo itself, so take it with a grain of salt, but it is promising roughly three times the battery life of comparable current-gen controllers.
Both GameStop and Amazon cite the figure (via Nintendo Life), which makes it seem official from Nintendo. The bad news? The new descriptions also confirm that it's not backward-compatible with Wii games. That means any games that use the Wii Classic controller will still need you to plug one into your Wii remote to play on the Wii U.