I would think they should post immediately, but I am not sure. A lot of people are complaining about delayed miles posting of all kinds, delayed medallion credentials, etc....with the massive IT mess Delta has, its not surprising. Make sure you keep all documentation for backup!
BTW, good call on canceling...that ticket should be no more then 150k each I believe for JFK-SYD at low level (Delta doesnt post SYD mileage requirements) However, very few people if any have reported getting SYD BE at low level...