Since the shuttle bus has a restricted schedule, I thought it might be interesting for others to post it here:

Weekdays
To Airport - 07:30, 08:30, 9:30, 16:15, 17:15, 18:15
From Airport - 08:00, 09:00, 10:00, 16:45, 17:45, 18:45

Weekends and National Holidays
To Airport - 10:45, 11:30, 12:15, 13:00, 13:45, 15:00, 15:45, 16:30, 17:15
From Airport - 11:15, 12:00, 12:45, 13:30, 14:15, 15:30, 16:15, 17:00, 17:45

The travel time is 30 minutes in each direction. The transfer to the airport should be reserved and costs 7 EUR. The transfer back to the hotel is free, on a first come first served basis and departs from platform A9 at the airport opposite the Sheraton.

The concierge informed me that the service is mostly
used by tourists which is why there are more weekend buses.