how do you know when your warranty is expired

The standard warranty is one year. Settings > General > About and you can check your coverage there.

Aestrada59 Said:

"How long is the warranty for the iphone 12 promax"

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Obtaining Warranty Info:

First, Find the Serial Number:

1. Tap: Settings
2. Tap: General
3. Tap: About
4. View: Fourth Line-Down

Then, Enter the Serial Number here: Check Your Service and Support Coverage -Apple Support. See it is comes up as an official iPhone.