If I were to make one definitive guess, I'd probably say sometime in 2024, as things will likely be slowing down for the Switch as far as new major releases and console sales by that point. That said, Nintendo really seems to be leaning into this being a much longer lifespan for a console than what we've seen in the past. A 2024 launch would already make this a longer lifespan than most Nintendo consoles, but I could see them riding the Switch out until 2025 if they have the games planned. I think it's a bad idea to just completely drop support for the console like they've done at times in the past. They need to basically do what they did for the 3DS at the end of its life, but I think it would work out better in the Switch's case than it did there. (given the Switch had already released by that point in the 3DS' case so none of those 3DS games sold well)
Personally I don't think there's any chance it comes later than 2025. Holiday 2025 is really pushing it, anything later than that is too late.