One of the privileges’ of the prime minister is to name the election date. The prime minister is required to provide 33 days’ notice. But beyond that, there is no legal requirement to let the public know. However, the latest possible date for a federal election is Saturday, November 30, 2013
Section 28 of the Constitution states that "Every House of Representatives shall continue for three years from the first meeting of the House, and no longer, but may be sooner dissolved by the Governor‑General."
(Melissa Clarke, on ABC News)