iPhone 15 Pro screen on time is only 4-5 hours

It depends on the usage of the iPhone.

Screen-on time on an iPhone refers to the total amount of time your device’s display has been actively lit up and in use — whether you’re browsing, watching videos, texting, or using apps. It’s a key measure of how much you’ve actually interacted with your phone, and it directly impacts battery life.

The iPhone 15 Pro was discontinued on Sept 2024, so you purchased a used phone that is over a year old with a failing battery.

Take it back and get your money back.