How to take off genuine iPhone displays message

Just after 15 days.

You will see a notification on your Lock screen for the first 4 days that you use your device and in the Settings app for 15 days. After that period, you can find the notification by going to Settings > General > About.

Extracted from the support article that led you here.

Regards

Giulio

As already stated, it will stay in Settings>App for 15 days.

Then you'll have it only in Settings>General>About.