I see what you mean.
What you need is a variable, named “AskForPassword?” or something and intialize it to true
Then when the user enters the password, it will set AskForPassword to false
and wait 900 seconds.
When the 900 seconds are up, it will set AskForPassword to true
The last thing you need is when the screen loads up, it checks if AskForPassword is true or false.
If it’s true, it will ask for the Password.
If it’s false, it won’t show it.
(Also, your English is fine! )