For student who lodged overseas, Australian immigration government claims to process within 12 weeks. But from our own experience, sometimes within one week.
For student who lodged in Australia, the period is decided by the course you applied. To most students, their visa can be issued within two weeks and sometimes as fast as a day.
However, there are two factors that may cause delay. One is that you apply during peak time and the other is the lack of application data.