When there is a majority govt. the party in power can choose anytime to call an election up to a maximum of 5 years. Usually the 4th year is the traditional time. The last federal election was Oct. 19 2015 so technically Oct 15 2020 would be the last date a federal election can be held. Expect a decision for an election in 2019 however.
Sorry Oct 19 2020 I guess is the last date for a fed election.