Nintendo announced they were making a new mainline Animal Crossing game on 13th September, 2018 (just under 2 years after the Welcome Amiibo update dropped):
We then didn't get any more information until E3 2019's direct, just under 9 months later on 11th June 2019, but we did get our first proper trailer, the title, and the release date of March 20th 2020:
There was then a second trailer on 4th September 2019, just under a year after the game was first announced:
The final thing we got before launch was the Animal Crossing Direct on 20th February, 2020, a month before the game came out:
So just over 18 months between first reveal and launch. Splatoon 3 has done something similar (revealed February 2021, came out September 2022) and has a definite life cycle of 2 years, so I'm assuming that if we're getting an Animal Crossing game after they're done with Splatoon, it's going to be revealed sometime this year... but that's assuming that it's either on the Switch again or we've heard something about the Switch's successor this year (although I doubt we're going to hear about the Switch's successor too early, because if it ends up coming out late 2024 the OG Switch's momentum ends up crashing way, way too soon as a result of the announcement. This is what happened with the Wii-Wii U transition, and we all know Nintendo wants to avoid that again at all costs)