Settings > Battery > Battery Health > Maximum Capacity
Expect the Capacity to decline an average of about 1% a month. That's an average. Some months it will not decline at all and some months it might drop 2%.
Expect the battery to last about 2 years.
It's time to replace the battery if Maximum Capacity is down to 80%.
Apple Support will not do anything unless your battery drops to less than 80% during the AppleCare coverage. If that occurs, Apple will replace the battery at no charge. Otherwise, they will say that your battery is performing normally.
Don't worry about Capacity week to week or even month to month. I check Capacity about once a quarter.
Plug the phone into the charger at night and leave it connected all night every night. Otherwise, charge the phone when it needs it.
More info here: iPhone Battery and Performance - Apple Support