There really isn't a "limit" to how long you're allowed to use your phone. It's all about how much you use it. If you're on it 99% of the time and your display is on the whole time, your battery life will be considerably less than if it were just sitting idle. 21 hours seems really good just sitting, 8-10 hours seems really good if your were using it non-stop. Medium usage would warrant a number between thos two figures. Like I said, it's all about how much you use it.