With luck, one hour would be the shortest and 2 hours if not so lucky.

Your baggage getting to the carousel usually takes longer than clearing immigration -though for Australians, the latter is automated if you have an electronic passport .

Most pax will have baggage checked in some way thru Customs often by Xray . If you were in J or F, you will get a priority customs clearance voucher which really helps speed thru that area.

It is then not a long walk(say some 500 metres) to the Qantas checkin in and security check.

Interterminal buses leave every 10 minutes or so and are quick - no further security checks at the domestic terminal. Once there you just follow the crowds up the escalator and you are right at the departure gates .

The only problems with connections is if the UA a/c arrives late or both the flights from LAX and SFO arrive at the same time - as it seems to take longer to get the bags unpacked.

All in all , in economy , easily make it in 2 hours .In the front cabins , 90 minutes no problem.

Qantas are usually pretty forgiving if you check in late at the fault of the arriving carrier.

Happy to give more info if needed.