how does the "Time since last full charge" update in iOS settings?
From what I've noticed, the Time Since Last Full Charge only resets to 0 min when you charge your device completely. If you, for example, had been using your phone for a day, put it to charge in the evening, but without charing it to 100%, the timer will not reset. If you do the same thing the following day, it would display "2 days", even though you have charged your phone last night.

And it also doesn't start counting the time until you unplug your phone from the charger.