https://www.thegamer.com/playground-games-fable-four-years/
So they have started working on this (fable) as a concept in 2017
He's the lead engineer of the studio, one of the few people working on a project since the very start of a project, since it's needed for budget estimations included in a game pitch
His work in the pitch stage of the project is to analyze if it is viable for them to make of this type and size, to design what they would need to build in terms of engine and tools, the amount of resources that it would need, the hirings needed in terms of programming, etc. to put them in the budget for the pitch of the game.
Once approved his team would make a rough prototype of some the tech they'd need for a vertical slice, and after then (with a bigger team but still way smaller than the one in full production) the vertical slice.
They did bought a new studio in late 2017 to work in the game and announced plans to hire around 200 devs to work there (in addition to the also grow their Forza studio). As I remember there was some news in 2018 that they did open like over 180 job offers to work in the Fable studio, pretty likely once they hired (takes around a couple of months after releasing the job offers) all the needed people is when they moved to full production. So they may have been in full production for around 4 years.
The average AAA takes 4 to 6 games to be developed, but some things make them need way more time: when it's a new IP for the studio, like this case. When it's a new totally new genre for the studio (things like quests, characters speaking, combat, etc. are new to them and their engine), like this case. When it's the first next gen game for the studio, like this case. When they build a new engine for the game, like in this case. When it's an ambitious big ass and detailed open world game with a lot of content, like this game. When a global pandemic happened during the development of the game, like in this case. When the team is understaffed for the amount of people that traditionally works in projects of this size, like in this case.
I think pretty likely this project will need at 6 or 7 years of development, maybe even 8 unless they outsource most of the related work (almost all AAA games have a big chunk of their development) and/or increase several times that size of around 200 devs.
I would be surprised if it gets released before 2025, late 2024 being optimistic. This is assuming the related changes, delays and other issues involved in games of this type but in the case that the project doesn't get major issues as would be the team not being capable to deliver a good enough game in this genre, which I bet won't happen. I'm confident that the development will be pretty smooth -I assume it will have the typical, common hurdles of many other AAA developments- and that they'll deliver a quality game.