I heard that Ishin was actually supposed to be pretty short, I gave my copy to a friend and he said he beat it with most of the substories in about 30 hours. I'm sort of hoping that Kiwami has the same kind of length, seeing as it's looking a lot like a reskin and tweaking of 0's graphics and mechanics.
I'm about to hit 100ish hours in 0, having finished most of the substories, and I've been playing it on and off since March, so I'm hoping that Kiwami won't burn me out too much before 6.
Ishin is definitely shorter in my opinion if your doing the main stuff, the story chapters are fairly short in the beginning (as usual, late game chapters are longer, action filled spectacles).
However if your going for 100% stuff, there are a few bits that will definitely take a long time - maxing out friend gauges is a real slog, the CP list (complete various mundane tasks over and over) is an insane grind and don't even get me started on grinding for certain materials...
In that regard to 100% the game it probably took me around 150 hours to finish the game which is fairly in keeping to my other Yakuza playthroughs. I think 0 took me 130 hours to 100%, it has far more stuff to do, but the completion goals are easier to achieve and the CP list is much less of a grind.
I also wonder though how long Kiwami will be. The original Yakuza only takes me 20-30 hours to do everything in, but the amount of mini games from the rest of the series plus the CP list returning will bump up that time somewhat. Then there is the question of how much new content they will add, going by the videos shown so far there are new bits added throughout the game so I'd guess it'll probably take me at least twice as long to finish the game this time round depending on the mini game completion requirements.