The iOS Usage menu in the General Settings menu can tell you how long it's been since you last charged your phone. Anytime after your phone has reached 100% battery, unplug it and navigate to Settings > General > Usage, and scroll to where it says "Time since last full charge". Add up the "Usage" and "Standby" times and you'll know how long it's taken your phone to discharge from 100% to your current battery percentage.