What is the Average battery lifespan of an iPhone?

Yes, that is perfectly normal.

Batteries degrade over time with usage due to chemical aging, that's just how they work.

You can expect about a 1% drop per month with average use on an iPhone. This is an approximation, that is why it may be plus or minus a few percent on a month to month basis.

Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.

iPhone Battery Replacement.

iPhone Battery Repair & Replacement - Apple Support

About Lithium Ion Batteries

iPhone battery and performance - Apple Support (IN)

It's not only OK, it's expected your battery health on average will decline about 1% per month of ownership. Battery health declines with every full cycle charge and that is not something you can stop. When the day comes that the health of your battery dips to 80%, which it absolutely will do someday, it will then be time to pay Apple to replace the battery. Until then, don't worry over something which is normal and cannot be stopped.

That is absolutely normal! All batteries degrade with time and usage. The typical Apple battery lasts about 2 years with normal usage. When your Battery Health reaches 80%, it will be time to make an appointment at the nearest Apple Store for Battery Replacement....

Apple Store - Find a Store - Apple

You're very welcome.

You're welcome.