This will give you a good idea of how long you can go portable with your handheld device, before it runs out of juice to power all your favorite stereoscopic 3D gaming beasts.
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To charge the battery all the way, it will take three-and-a-half hours. If your battery is fully charged, you can play games for anywhere from three to five hours before the battery runs dry.
Your mileage will vary though depending on how high you have the 3D effects turned on, whether or not you are playing with sound, and how strong the backlighting is.
If you chose to play a classic DS game on the 3DS, the charge would last you between 5 and 8 hours.
Naturally, having the backlight on will take more juice as will playing with the 3D effects of the system (which can be turned entirely off via a slider).
The 3DS will be released in Japan on February 26, 2011 and in March 2011 in America and Europe.
Here is the battery-life breakdown for 3DS:
You can see the 3DS and it being charged in this Japanese promo video.