Any talk of "Dubai returns" and turn around times etc. is a complete waste of time. Both aircraft arrive into Dubai and then continue to either Melbourne or Sydney
A 20 or so hour slip as you suggested is probably likely. That would mean trips would be shorter and worth less hours. Thus more trips on an 8 week roster.
Back in the days when the LHR base crew flew to HKG, Qantas were able to roster a 20 hour slip because of the way the schedule worked in and out of Australia/UK. After a while this reverted to a 44 hour slip due to crew fatigue issues. However the Dubai flight are obviously a lot shorter so I doubt the company would apply the same benefit to rostering these trips