In this reality, Charles Xavier became the Juggernaut who formed the X-Men. Thunderbird is a member of Charles Xavier's X-Men who serves as a police force that aids the mutants and persecutes the sapiens.
The total flight duration from Sydney, Australia to Boston, MA is 20 hours, 41 minutes. This assumes an average flight speed for a commercial airliner of 500 mph, which is equivalent to 805 km/h or 434 knots. It also adds an extra 30 minutes for take-off and landing. Your exact time may vary depending on wind speeds.
While Australia is a fair distance away, the flight is not nearly as daunting as you may think. The flight from Los Angeles to Sydney is around 14 hours, and the flight from Los Angeles to Melbourne is around 15 hours. When travelling to Australia from the United States, Los Angeles and San Francisco are your only 2 departure options.