I guess towards the end of the year. That's when Ridley's current movie's finished.
From Imdb: All The Money In The World release date: USA 16 November 2017 (AFI Fest) and USA 8 December 2017.
Give or take 1,5 year to make the next Alien movie, brings us to 2019, the supposed release year for Alien: Awakenings
More precise, it's May 2019, which could be possible since Covenant was released in May as well.
Micheal Fassbender has two upcoming projects: The Snowman (post-production) 2017 and X-Men: Dark Phoenix (filming) 2018.
So it's possible he's available around the time All The Money In The World's finished.
And there's no mention of any other upcoming movies on his Imdb page, which I find a little odd for an actor of his calibre.
So fingers crossed...