With the screen's backlighting turned down (but not off), you'll get 3-5 hours of battery life while playing a 3DS game. That number extends to 5-8 hours if you're playing an older DS game on the handheld.
Of course, the more you increase the screen's brightness, the bigger the hit the 3DS' battery life will take. While people will be upset that these numbers seem low for a Nintendo handheld, they're not that far off what you'd expect from a DSi, and considering the 3DS is a much more powerful device, all that extra grunt takes its toll on a battery.
[Nintendo] Kotaku]