So Yakuza 6 is set to release in Fall 2016 and Yakuza Zero was released in March so it's safe to say development started somewhere in April or May so by the time Yakuza 6 will be released if there is no further delay's, it would have been in development for around 16 to 18 months. Hopefully it will be delayed atleast till christmas 2016. Even if it isn't, i'm pretty sure they have enough time to work on most of the feature's and refine them and if not, they can always delay the game.