Check the ALSA bus timetable before you land, not after
The Line 245 airport bus runs on a fixed schedule. Not on demand, not continuously. There are roughly 8 to 10 services per day in each direction, and they do not all coincide neatly with flight arrivals. If you land at an odd hour and the next bus is 90 minutes away, you will either wait or take a taxi for €25–30. Spend two minutes before you leave home checking the current timetable at alsa.es for your specific travel date. The schedule changes seasonally.