I think it's going to be in 2018.
Here's what is going to happen:
I don't see them doing a history (classics) tour after the huge success of The Book of Souls. Bruce also said they wouldn't do another history tour, so we got some official word about this. The Book of Souls World Tour will probably end on first months of 2017 and maybe then they will do a short summer tour on Europe with classics/random (but ofc not so random) setlist. By the end of 2017, they will enter studio to record their last album. Their last album will be released in 6th july 2018, followed by a massive world tour which will happen until the end of 2019. Between 2020 and 2021, there will be the Farewell World Tour, which is going to be their last. The setlist is going to be a mix of their 17 albums, with Alexander The Great representing Somewhere In Time.
That's it.