3 hours give or take depending on the gamepad settings. I recommend buying the high capacity battery. It doubled battery life from about 3 hours to 6 hours. If you need more battery life there's also the Uboost but it adds weight to the system.
https://store.nintendo.com/ng3/browse/productDetailColorSizePicker.jsp?productId=prod150204
http://www.amazon.com/Uboost-Wii-U-Nintendo/dp/B009Q5XGO2/ref=pd_bxgy_63_img_y
http://backloggery.com/kdog254 http://www.twitch.tv/kdognumba1
3DS: 1676-3698-5986 PSN: jotaroxtreme Steam/NNID/Ouya/XBL: kdog254