You should consider replacing your iPhone's genuine battery when it starts to significantly impact your daily usage. This typically occurs when the battery's health deteriorates below 80%, meaning it can no longer hold as much charge as it used to.Signs that your battery needs replacement include:

Reduced Battery Life: Your iPhone's battery drains much faster than before, requiring frequent charging throughout the day.

Unexpected Shutdowns: Your iPhone shuts down unexpectedly, even when it still shows a significant amount of battery life.  

Battery Bulge: In some cases, a deteriorating battery can cause the phone's back to bulge.  

Replacing the battery with a genuine Apple battery will restore your iPhone's performance and battery life to optimal levels.

It's recommended to have the replacement done by an authorized Apple service provider or a reputable third-party repair shop to ensure quality and safety.