Hardly any time on the phone most days - perhaps 15-20 minutes.One thing that can drain batteries quickly is simply using the iPhone as a phone. How much time you spend on actual phone calls?
Did Apple replace your iPhone 6S with another iPhone 6S?
And, have you checked the Settings > Battery > Battery Usage by App to see what that says?
Yes. Another iPhone 6S.
Nothing stands out in the battery usage section: last 10 days:
Safari 17% (2h 13m on screen)
Mail 15% (1h 42m on screen, 28m background)
Calendar 7% (8m on screen, 46m background)
Phone comes in 5th (1h 18m on screen)
(Actually, it would be interesting to see what comes in at top of other users’ lists.)