Update: A couple of more details: Battery life for DS games is five to eight hours. It will take three and a half hours to charge the 3DS.
On a number of occasions, Nintendo has stated that they would like to make the battery life of the 3DS comparable to that of the DS. However, it does not seem as though they were able to do so. According to Nintendo’s newly updated 3DS site, the battery life is listed as approximately three to five hours. This is quite different than the DS, though you have to wonder if different features will affect its performance. For instance, will turning all Wi-Fi capabilities off increase the battery life?