If you have registered to receive departure alerts, you will only receive an alert if departure is delayed by more than 30 minutes. If the new estimated departure time then changes by a further 30 minutes, you will again be notified and so on. You may also opt in during registration to receive an additional alert informing you of the real-time flight departure.
The same applies if you have signed up to receive arrival alerts.
Email alerts are sent 24 hours a day, depending on the status of the flight.