The Prime Minister could ask for a double dissolution election (an unusual type of election - it's happened six times in Australian history - sanctioned under the Constitution to deal with Senate obstruction of government Bills) at any time which would mean an election as little as 33 days after he calls it (the minimum 'campaign period' under the Constitution). If he doesn't do that, an election is likely about a year away - there are practical reasons why a non-double dissolution election before 6th August 2016 would be unlikely (though nothing constitutionally prevents it) and an election must be held by 14th January 2017 at the latest.