First charge doesn't have to be 12 hours, just a full charge.
It can be charged on any charge.
Once a week * there is a complete draining and charging process. So it goes from red to full charge.
The iPhone xs max has the function of over-charge protection for the phone, but it is recommended to unplug the phone after it is full, and it is not recommended to charge for a long time, such as overnight charging.
When the phone battery is too low, you should try to start charging in time.