What is the duration of visa extension if a student wants to pursue a 1.5-year long course? Will the extension be of 1.5 years only?

In the first place, I do not know as to who recommended a 1.5 year course to you. If you do this then you will not be eligible for a Post study work visa, the criteria for a PSW is that you should join a course of a 2 year duration which you can complete in 1.5 years if you are capable and want to d