I knew nothing at all about these Yakuza games before they came to GOG. I started with 0, and so far I've played up to the start of Chapter 3 in that game.
And that already took me 45 hours in-game, but I spent a lot of time doing mini games and sub-stories and grinding out random fights in the street to level up my abilities, and also grinding out completion points by eating all the meals in many restaurants, etc.
Based on my experience so far, this seems to be an essential key point:
Munkee79: but if you're going to only ever play one game in the series (an entirely feasible scenario for most people) it should be 0.
If you intend to do a completionist-style run of these games, then Yakuza 0 will take you a very heavy investment in terms of the amount of literal real-life time that you will have to invest into it, and I assume some or all of the other games may also follow suit.
So with that in mind, you have to think about how much time are you actually, for real (not just in theory), going to be able to invest into these games.
And if you want to do everything, or most things available to do in the games, then maybe you won't have enough time to invest in order to play, much less complete, all of them, or maybe not even more than one of them.
So maybe some players shouldn't even assume that they are going to get to all of the games, as I would imagine that for many people, that simply won't be possible due to the sheer amount of time it would require (especially if they intend to complete all the content of every game).