First or Business you will very likely need to be ready to book 355 days out (you cannot book any further in advance). So you will need the points in the next 6 months to book a flight in 18 months time.

Currently for MEL-LAX there are a few days 330-355 days out with First or Business availability.

It is not true that availability depends on status as posted above. Upgrades are processed based on status, and Platinum/Platinum One customers may be able to have seats released.