I have found the Marbella bus timetable valid since October 2013 and is:
Laboral days from Marbella to Malaga airport:
7:15 8:15 9:15 10:15 11:30 13:15 15:15 15:30 17:00 19:15 21:15
Laboral days from Malaga airport to Marbella:
8:15 10:15 11:30 12:00 12:30 14:15 16:15 18:00 18:50 20:15 22:30
Weekends and holidays from Marbella to Malaga airport:
7:15 8:15 9:15 11:30 13:15 15:15 15:30 17:00 19:15 21:15
Weekends and holidays from the airport to Marbella:
8:15 10:15 11:30 12:30 14:15 16:15 18:00 18:50 20:15 22:30
Source:
http://www.xiicongresotrabajosocial.es/ ... ctubre.pdf