The actual completion time depends very much on the builder, the suppliers, the sub-contractors, the weather, how well they stick to the plans, what holidays are scheduled during the construction, etc.
It is possible to build a house in 6 weeks and I've seen it happen with the bulk builders when they're building a standard house. They do it very quickly (and cheaply in more ways than you probably want to think about).
For most stand alone builds they contract for 8-12 months but they often get it done in 4.
The most recent triplex I can think of took about 8 months to build. They had a 2 month delay over Christmas.
Another factor is to consider if you want to subtitle the land after the construction. A 2 month time frame in VIC is not uncommon for this but it can often take a lot longer.
We see most off-the-plan projects get delayed on an ongoing basis, even after the construction is completed (this would be due to waiting for the council to strata title them). In a recent case we were told to expect settlement within 4 weeks (that was back in May). The building was completed in June and we're not hoping it will be ready to settle by November.