danieljay:
If it is the same, you need to compare the time since the change (Hint: Millis() - timeSinceChange) to see if it's greater than a certain interval.
i want to summarize that when this sensor value has not changed over a specific period of time, do something. I'm not understanding how what i quoted will do that. i may be wrong. i can't visually understand it.
Here's an earlier project of mine that implemented pretty much exactly what you are asking:
If the garage door is left open for a certain period, action is taken.