If that's the case then they worked longer on DOA5 than 6. But that makes sense since everyone got completely new designs etc.
DoA5 was announced in 2011, only about 10% completed. Basically the core was built and they just had to work on the characters. It was released in 2012 so probably around 18 months if they were already working on it prior to D.i.D #7 tournament.
DoA6, announced this year, coming out in 2019. Development probably started near the end of 2017, so roughly the same time frame.
Yeah... I don't buy that. For any game, the base is the hardest part. There is no way the base is only 8% of the whole game.
But never mind. Getting into discussions of percentages is a waste of time. To discuss things that are important;
I prefer for them to take longer and release a full feature rich game, rather than releasing DOA6, and re-releasing a bunch of iterations afterwards. Don't repeat DOA5 please.
Who said the base was only 8% done? They probably started working on the base/core engine of the game several months ago. Once that is completed, actual development on the game begins. Granted, this is only speculation, but from following fighting games for years and seeing how fast they come together after being announced its safe to say that they advance pretty quickly once the core system is in place. The rest is adding characters, playtesting, balancing, and bug testing.