Battery life

Hello ~ Usually you would need to have the battery replaced when it drops to 80% or below. Take a look here here for more information about replacement and cost:

iPhone Battery Replacement - Official Apple Support

Also take a read through this excellent user tip by *Lawrence Finch* for further battery info:

When to charge your iPhone or iPad - Apple Community

~Katana-San~

There's really no accounting for how long a battery is going to last. Sometimes it's random. However, Apple's rated life for all current iPhone batteries is 500 cycles to the point where it has 80% of the nominal new battery capacity. Some do a bit worse, but most likely do slightly better.

There are just too many variables to predict how it will work for you. I recommend not getting too hung up on the battery health reading as we see a lot of people who get obsessive and they can't seem to enjoy using their devices. If the battery needs to be replaced, it's a small cost relative to a new device.