Hi MON05,
I had my day with Qantas last week, started at 8am (i would seriously recommend coming down the day before and staying at the hotel if you have far to travel).
Some people will be sent home after the morning session while the others get through to interview. If ou don't get through then you can be out of the hotel by about 1. If you get through then expect to be there until about 4.30/5 depending on how far down the list of interviews you are. You'll have to do a quick computer test while you wait for the interview too.
I would suggest not booking your return flight to early as you woulnd't want to run fro the Jury's inn...it's about 5 min fast walk to Hatton Cross tube and then you've got to leave time to get to the terminal.
Hope that helps