Route option 1 The scenic coastal route
Take the M1 west to Geelong, then the Surf Coast Highway (B100) south through Torquay, then the Great Ocean Road (B100) all the way to Warrnambool. This is the slow, beautiful route — you drive the entire length of the famous road.
Use this route on the way out, on a multi-day trip, or any time you actually want to see the road. It takes ~4 hours to the Twelve Apostles (without stops) and ~5 hours to Warrnambool.
Pros: every Surf Coast town, every viewpoint, the cliff section, the Otway forest. The reason you came.
Cons: long. The road is 80 km/h limit but the bends mean you average closer to 60. Tiring for the driver.
Route option 2 The inland fast highway
Take the M1 west all the way to the Princes Highway (A1), then west through Colac and Camperdown to Warrnambool, then south to Port Campbell or the Twelve Apostles. Fast, mostly straight, dual carriageway for much of it.
Use this route on the way home from a multi-day trip, or for a 1-day trip's return leg. It takes ~2 hours 45 minutes to the Twelve Apostles direct and ~3 hours to Warrnambool. You miss almost everything beautiful but save 90 minutes.
Pros: fast, predictable, easy on the driver, dual carriageway from Colac onwards.
Cons: passes through forgettable agricultural land. You can't stop and say you've seen the Great Ocean Road if this is all you've driven.