From Sydney to Rome, it is about 16315 kilometres (10197 miles), and a flight would take about 17 hours and 57 minutes.
However, 20 hours is more accurate.
Why not call the Airline you will be travelling on and ask them. What better way to find the answer to your question. Now, why didn't you think of that.
Assuming a direct flight (aprox. 16000 km), and an average speed of mach .8 (average cruising speed for a commercial jet) the flight time would be somewhere around 17 hours give or take a few minutes...