I'd give a totally dead battery at least overnight. And don't let it drop below 40% after that. Lithium batteries shouldn't be discharged any further than that if you want them to last for a while. (Lead acid batteries can be, but the battery for a phone would weigh a few pounds. The industry uses lithium despite the problems because it's the lightest metal [at room temperature - hydrogen is too, at almost absolute zero, but the cooler would be bigger than a car.)