I wouldn't take that literally - they've already lost a few days for the fire and I suspect will be looking to complete at least two weeks before Christmas should any quarantine rules be in place then. I suspect it'll be around the same number of days as the last series, which itself was cut short towards the end.A 7news article about the 2020 upfronts said this:
"The master game player returns in 2021, welcoming a new group of Housemates who will live together for over two months, cut off from the outside world in a custom-built house fitted with cameras and microphones recording every inch of the house and their every move, 24 hours a day."
So does that mean at least they'll be in the house for 8 weeks + ?
Also from the upfronts worth noting they've said Holey Moley (which I think will flop) will air first so doesn't look like Big Brother will go up against MAFS from February anyway, though it may come in to the schedule a few weeks later.