It's the Great Ocean Road
If you're not really stopping to catch all the sights (other than a quick walk at the Apostles and Loch Aird), you can do it all in one very long day (not returning on the GOR). With somewhat limited stopping, two days.
Base this on: 2 hrs Melbourne to the start of the GOR at Torquay
2 hours to Apollo Bay
4 hrs to Port Campbell
Head inland and to Melbourne from there (I've done the GOR twice but never went back to Melbourne). 4 hrs?