Well, if we look at Blomkamp's previous 2 films:
Elysium- Bought by Sony in January 2011 based on concept art, though I'm not in what state the script was in. Production began in July 2011. Originally set for a release date in late 2012, but reshoots were still happening as late as October that year, so the release date was pushed back to March 2013, then into August 2013. Lots of delays in that film. And during the latter part of post-production,Blomkamp did some work on CHAPPIE(sort of like he did with Alien 5)
CHAPPIE- Filming began in October 2013, completed in February 2014 with reshoots done in April. Movie was released in March 2015.
So figure with Alien 5 we have at least 6-10 months of pre-production, 4-5 months of shooting and a year of post-production, given previous Blomkamp films. So about February-July 2017 for production to be complete, I'm going to guess Alien 5 will hit theaters in late-summer/early-fall 2017, though the studio may decide to delay its release into a better release slot in 2018.