When I was using the 6s as I normally would -- sifting through threads on Reddit, watching YouTube videos and playing the odd game or two -- the Smart Battery Case lost between 15 and 20 percent of its battery life in an hour. As such, I usually got about five to six hours before the case died completely and the phone switched to its internal battery. After that, the iPhone 6s ran on its own power for just under 13 hours on average, for a total of 18 or so hours of near-continuous mixed usage.