As part of its big Nintendo World event being held in Tokyo at the moment, Nintendo has released a chart outlining the final hardware specifications for the 3DS. Most importantly, that includes details on the handheld's battery life.
It takes 3.5 hours for a 3DS battery to fully charge, which is pretty good. As for how fast that battery drains, Nintendo has only provided a select number of figures, all based on the presumption you're using the handheld on a lower screen brightness level.
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.
If you're after definitive battery life tests on a unit under all kinds of circumstances, we'll have to wait until people can use one without Nintendo employees standing over their shoulders.