Depending on where you stay in Amini Island, a free airport shuttle might be a round trip to and from the airport or it could be just a one-way shuttle to your hotel.
How often will the shuttle run?
It depends on the hotel you choose to stay in Amini Island. Some hotels have shuttles that run at regular intervals day and night; others require advance reservations for custom departures and pick-ups. It's important to check the hotel's policy on Expedia before you book.