Presuming that the next Federal election is a normal house and half Senate election then there is a requirement to have a half Senate vote by a date in May 2019.
There can't be a normal election held before Saturday 4 August 2018.
Complicating election planning is the Victorian state election on 24 November 2018, and the NSW state election on 23 March 2019.
So maybe a Federal election campaign held over Anzac Day and Easter 2019.
Also a possibility of bringing the election forward to August-September 2018.