Nintendo's new hybrid console is rated for "more than six hours" of use away from an outlet, but mileage will vary depending on the game. In a press release sent out following last night's Switch event, Nintendo says launch title "The Legend of Zelda: Breath of the Wild can be played for roughly three hours on a single charge." Obviously, less-taxing games will last considerably longer, allowing Nintendo to claim six hours of battery life with a clear conscience.
Whenever the Switch is docked to the TV it automatically charges, so you should leave the house with a full battery every time. However, the Switch uses regular USB Type-C for charging, so you could feasibly top up with the same charger you use for your laptop or phone.