If you are flying into Sydney, the flight will most likely leave around 23:00 hours. You need to check in (at least) three hours before that. The queues here can be long and arduous.
From my experience, getting the car to the rental place by 18:00, then wait for the shuttle bus to the airport - none of the rental places are actually right at the airport - and deal with the queues and security at the airport, you will not have much time left to even check out the airport shops. And remember, there are two security checks to go through, just as there were in Sydney.
On a week day, I would leave at least 45mins to get from the Hollywood area to LAX, traffic holdups are not rare.
Better to be there early than late.