Flying from Bangkok to Athens will be a long motherfucker and flying back to Oz more so. Between waking up in Brisbane to get the train to the Airport, then to Sydney, to Houston Texas to San Francisco to my own driveway was 28 hours, the single longest flight to from that I have done.
Try to minimize layovers without running the cost too high. The time frames from Bangkok to Athens I see are from 17 hours to 32.