The Parliamentary Library has published a research paper setting out Federal, State and Local election dates over the next few years.
According to the paper, the earliest possible date for a joint House of Representatives and half-Senate election is August 4, 2018. The last possible date for such an election is May 18, 2019. Assuming no unforeseen events, the next federal election is almost certain to fall between these dates.
The last possible date for a double dissolution is May 4, 2019. A double dissolution of the parliament cannot be called later than February 27, 2019.
The paper shows that there will only be two state or territory elections over the next eighteen months: in the ACT on October 15, 2016 and in Western Australia on March 11, 2017.