It's an interesting question.
From Bayonetta to Bayonetta 2, there is a 5-year gap. Obviously, they didn't start production right after they finished the first game, and Nintendo got their hands on it mid-production. I can't see for definite, but my guess is that Bayonetta 2 took 3 years of production.
Bayonetta 2 consisted of a prologue, based in New York, followed by Noatun, which was a whole new area. After Noatun came the big water section, also new, followed by the big Insidious that you get sucked into. This goes over seven chapters (to my memory).
Then, Gates of Hell. In this section, it's half new terrain and half Paradiso, in Paradiso there are elements of previous areas in the game, including where you first meet the Urbane enemy and the location Loki makes his first appearance. This takes up two chapters.
You then have Hell, which is a new area and covers three chapters. After Hell go back to Vigrid from the first game for three chapters before the final Aesir battle which is a whole new segment after a big section on an assault plane.
I say all this because Bayonetta 3 may take the same idea of use and reuse segments. If this is the case, Bayonetta 3 could take about 3 years, in which case I'd guess we'd see at least some gameplay by the end of 2021 or in early 2022. If they decide to make an entirely new game with no reused segments it could be, at most, another two years, by my guess.
None of this has taken COVID into account. Because I don't know Platinums situation, it could be that development was put on hold for a little while, or it could be that development went much slower due to a reduced team working in offices.
And, keep in mind, that this is a Nintendo project. Nintendo will keep smacking a project until it is in perfect shape and if they don't like something, it is ultimately their decision to bat it away and ask for a different approach. It also would have been their call to announce the game when they did, and my guess is that the announcement was exactly what it said it was: Bayonetta 3 is in development. They didn't actually say anything else, meaning it's entirely possible that all they had at that moment was a basic plotline and a short animation to show at the Game Awards.
I obviously can't make concrete statements. I have no connections to Nintendo or Platinum, I have no contact with any leakers, I am purely working off assumptions and guesses, so take this with plural grains of salt, but need I remind you that Hideki Kamiya did say to forget about it earlier this year.
It could be a while.