I doubt we'll have to wait til 2020. Most probably it will be published next year, maybe towards the end. UNLESS they are stuck with some insourmountable mechanical problems they find difficulty in solving and the issues cant be ditched -like not having flying cars in the game, if they want them in the game, or IDK, customizable/2 gender characters, or smthg as major as this-, I cant imagine the development process lasting more than 5 years...
Only the super major studios (like Sony) can afford doing that (and sometimes they did it, sucessfully, with The Last Guardian, for example). But the large sums of money CDPRED is still making with Witcher 3 wont last forever, and the studio has now between 350-500 people working in the baby... Thats a lot of dollars.
In a recent Polish interview, Michał Kiciński, co-founder of CDP hinted -actually it wasnt him, but other guy who made the remarks) that the time from actually announcing the launch date and the launching per se could well be less than a year, with the caviat that huge sums of money should be invested in publicity, in a relatively "short" period of time. Of course, this should not be taken as anyhing more than rumours, but maybe they are going for a different economic model of development/advertising.