It will take almost 3-4 months for interview after written test.
Focus on security related terms in interview preparation like what is IED, different security concerned areas of airport, Bomb threat or hijacking dealing situations etc. And about your personal and academic qualification. Some questions from geography.
ASF prime duty is to safeguard airport and installation there. They conduct search of passangers. ASF inspector has power of SHO according to law but it is not exercised practically due to some reasons. It is very courteous job, you will have to compromise on many things to remain courteous. ASF hand over the culprits to the concerned agencies like police, FIA. No self action.
Training of 6 months in karachi and starting sallery is around 29k.