So Nintendo unveiled the official numbers of the play time per charge for their new 3DS that will hit US stores in March. The play time for a 3DS title is 3-5 hours and 5-8 for an old DS title. It'll take up to three hours 30 to fully charge the battery. The numbers for the older versions play time are:
DS 10-14 hours, DS Lite 15 to 19 hours, DSi 9 to 14 hours and DSi XL 13 to 17 hours.
But with the new 3D features it really is inevitable to charge it more frequently.
And it's still better than the battery life of my PSP Slim that won't even last 5 hours of pure gaming. :/