how to detect change of state within a period of time.

Projects General Guidance
1

how to write a code if the button state won't change within 5 seconds LED will light up.

this is continuously checking whether HI or LO, ive already research about millis() function but cant figure out the use.

I know where to put the code but cant create the statement. please help

2

Let's suppose that you want to detect a change of state from HIGH to LOW

Each time the HIGH to LOW change occurs save the value of millis() and set a boolean to true. Any time that the input is LOW set the boolean to false.

Each time through loop() if the boolean is true check whether 5 seconds has elapsed since timing started. If so then set the boolean to false to prevent further triggering and run any other required code

3

Something like this pseudo code

void loop() {
  buttonState = read button
  if (buttonState == wrong) {
      mostRecentWrongStateTime = millis()
  }

  if (millis() - mostRecentWrongStateTime > desiredInterval) {
     // button has been in the correct state throughout the interval
  }
}

...R