If you want to test that an input remains in the same state for a period then using millis() for timing is the way to go.
Save the value of millis() when the input goes into the required state and set a boolean to true. If the state changes then set the boolean to false. Each time through loop() test whether the boolean is true and whether millis() minus the start time is greater than the required period. If so then the input has been in the required state for the whole of the period.