Inland Sydney to Cairns is 2400km and if you go inland route not alot to stop for really...
Then take a leisurely trip all the way to the tip.
For the run back to Cairns, well the development road is in a sweet condition at moment. Friends from Cooktown just came and visited us while working in the straits, they did Cooktown to Seisia in 8 driving hours... so add 3.5 to Cairns makes one long day
Depends what you want to see and do.
S