Looking back over recent series could be anything from 40-65 episodes for The Block over 10-12 weeks. I would bet on it starting the night after The Voice final - though I know with Aussie TV even a Grand Final can take place over weeks rather than a couple of hours!
I just don't get why Aussie TV is so cagey, launching shows with less than a weeks notice. Here in the UK they have to lock series in 10-16 days before broadcast, while the most competitive market in the world shows their hands months in advance. We know the exact time and date some series are starting on US TV not just in September, but also early next year, yet don't know about a series which could launch next week in Australia.
P.S. This goes against the principles of Big Brother but it would be quite interesting to run half the series this year, then let the HGs out for a couple of months, then put them back in for the second half of the series in Feb/Mar. Would be interesting to see how they change.