The airport bus has three direct routes. Ibiza Town #10, San Antonio #9 and Santa Eularia-Es Cana #24. The Ibiza route runs all year round while the San Antonio and Santa Eularia-Es Cana operate only in the summer months from June 1 to September 30. A full schedule of Ibiza Bus timetables are available here. Please be aware that Saturday, Sunday and Bank Holiday timetables are different.
Also make sure to check that the service is in operation during the time period of your travel. The Airport San Antonio #9 and Airport Ibiza Town #10 operate 24 hours while the Airport – Es Cana/Santa Eularia route operates from 7am to 11pm only. Outside of the summer months, a visitor would have to get the #10 bus into Ibiza central and change there for routes all around the island. All timetables and route information can be viewed here.