I did everything from 2004 to 2012 about twice for the sciences and maths in the month lead up to HSC. That was enough for me to get over 95.
However, it is subjective to the individual. You should know your content very well before doing past papers. If I had my time again, I probably would have done harder questions and solidify harder content then do past papers once or twice each. That way I could have gotten more. But yeah I got way more than what I wanted/needed so I don't care.
It's good to find a habit of what to do in the lead up to exams. It is specific for each person and for me I have improved my method of study in the lead up over the past few years.