If your aim is to travel Australia by seeing all there is to see and experience all the activities there are to try, then you really need to spend as long as possible travelling. Three to six months is the norm for working holiday makers while some people travel for years. It really is up to you. You should take into account actual travelling time to places (and recovery time) when planning your trip/s.
To help you make the most of your trip, check out my map following. It shows major attractions including the activities available there and how long they can take. Once you have a rough time-frame for your trip you can work out how to get around by linking the available options. Read blog Top 5 travel options to travel Australia to help you decide how you will travel Australia.