At what percentage should I replace my iPhone 11 pro Max battery?

sumairahmed ~ You could also consider replacing the battery when you notice "performance management" effects such as:

• Longer app launch times
• Lower frame rates while scrolling
• Backlight dimming (which can be overridden in Control Center)
• Lower speaker volume by up to -3dB
• Gradual frame rate reductions in some apps
• During the most extreme cases, the camera flash will be disabled as visible in the camera UI
• Apps refreshing in background may require reloading upon launch

[Source: iPhone Battery and Performance]

You can find Apple Support Articles such as the above by searching Apple's Knowledge Base.

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As a general rule, replace it at 80% or less. For more info, follow this guide: https://support.apple.com/en-us/HT208387.

Replace it at 80% or less.