4 hours
about 23 hours, 15 hours from LA if the tail wind is good 13-14 hours
probably a day.
from Melbourne to rome is 22 hours and 45 minutes
Melbourne (MEL) to Bangkok (BKK) Shortest Flight Duration 9 hours 20 mins
The distance to cover is around 1700 Nautical Miles, which takes a jet airplane around 4 hours to fly.