Nvidia employee acknowledges Tegra 239 SoC rumored to be powering Switch 2

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Switch was unveiled for marketing purposes on Oct. 20th, 2016 and launched on March. 4th, 2017 ... which is like 4 1/2 months of actual real marketing. I don't think Nintendo announcing NX in 2015 really means anything, they give zero details on the product or wouldn't even comment what kind of system it was or what games it would have, it was meant to appease stockholders that something other than the Wii U was coming and don't dump the stock please.

Switch Lite was unveiled July 10 and released on September 20th, 2019, basically 2 month gap there.

Switch OLED was unveiled July 6th, 2021 and released October 8th, 2021. About 3 months.

PS5 was unveiled on June 11th, 2020 and launched on November 12, so basically almost 5 months exactly from reveal to product launch. That's actually probably a more equivalent relevant for Switch because Switch (unlike the Wii U) is actually successful like the PS4 was, you don't want to announce a successor like 1+ year in advance or cut off a holiday season for no reason, that's just stupid. 

You really only announce game systems way in advance these days if you're existing system isn't doing that great (Wii U, even XBox One). If you have a very successful console you shouldn't be revealing a successor system or even a new model very far in advance any more because all you're doing is killing your own sales momentum, if you don't have sales momentum, then sure, announce away.

4 months even is plenty to build hype. All you really need is basically a good launch trailer and good launch titles these days, that basically is all the Switch had, Nintendo went from having a shitty brand rep with Wii U and even the 3DS not doing that great to only needing 4 months to build big hype for the Switch. It just takes a good launch trailer and a big ticket launch title and you're off to the races, it's not that much more complicated than that.