A:AnswerIt depends on the capacity of your device and the power powepack.
I have a Galaxys4 that has a battery capacity of 2800mah and this powerpack has 2200mah. I get almost one full charge.
A:AnswerI don't remember letting mine drain down to empty but my experience has been an hour or two. It's easy to plug it in at night so it will be ready the next morning.