Some institutes offer short term certificate or diploma courses in Airport Management that last about 6 to 12 months. Other institutes may offer a full fledged bachelor's or degree course in Airport Management or Aviation that lasts around 3 years.
One reason for different durations is course type. Shorter courses are usually certificate or diploma level. They give basic training in airport operations ground handling customer service cargo handling security and other essential airport functions. Longer courses (like a bachelor's degree) go deeper. They cover more theoretical knowledge management regulations industry norms perhaps even aviation law logistics airline operations giving broader preparation for higher responsibilities.
Another reason is curriculum structure and training methodology. Some institutes focus more on practical training, internships real world airport exposure which may extend the duration. Institutes also differ in how they schedule classes breaks assessments or how quickly students complete the course depending on their performance.
Finally institutes may design the course based on their resources expertise and the kind of qualification they want to offer - certificate, diploma or degree which influences how long the course runs.
If you want to join the aviation industry quickly perhaps as ground staff customer service cargo handling airport operations then a short term 6 to 12 months certificate/diploma course is a good choice. You can get trained reasonably fast and enter the workforce.
If you aim for more advanced roles maybe management airline operations regulatory work or want deeper knowledge then a 3-year degree course in Airport Management gives you stronger grounding and better long term prospects.
Thus the Airport Management Course Duration you choose should match your career goal time availability and ambition in aviation.