The 3.7V 1300mAh 5Wh Li-Ion battery provides a measly 3-5 hours of battery life. The sweet 3D action comes at a price. Even when playing original DS games, the 3DS lasts only 5-8 hours on a single charge.
The battery life is most likely diminished by the reported faster processor, dual LCD elements in the top display (to control the parallax effect), and more hardware intensive programs.
With the short battery life in mind, Nintendo ships the 3DS with a charging cradle for convenient charging. According to them, a full charge takes 3.5 hours.