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