You're browsing the GameFAQs Message Boards as a guest. Sign Up for free (or Log In if you already have an account) to be able to post messages, change how messages are displayed, and view media in posts.
How light you want the screen to be also makes a big difference in how long the battery will last. So if you make it pretty dark it lasts for a long time. Well that's how the DS is, but I'm pretty sure the 3DS will be the same.