Unless they plan on having the first year or two of Switch 2 games be cross-gen, then not very long. Nintendo has generally not supported their consoles long after replacing them with the NES & SNES being the only ones to have a few notable Nintendo titles after they were replaced. Meanwhile, the N64 and Wii didn't get anything after their successors were released. The last first-party games the GameCube and Wii U got were cross-gen Zelda games. The Game Boy had Pokemon Crystal released very shortly after the GBA was released. The DS had Pokemon Conquest and Pokemon Black 2 & White 2 released for it after the 3DS came out. The GBA had Pokemon Mystery Dungeon: Rescue Team and Mother 3 (the latter being Japan-exclusive). The 3DS did have a fair amount of support after the Switch came out, but the Switch was only a kinda-sorta "yes, but not really" replacement for the 3DS, and was a more directly replacement for the Wii U.

Given this track record, we might at best get one or two notable Switch games released after the Switch 2 comes out, again assuming Nintendo doesn't do what Sony & MS did with the PS5 & XBS and have a bunch of cross-gen games early in its life.

Until it sales 160 millions units!

Nintendo understand the value of their games, as exclusives to push hardware sales.
They typically dont support past systems for long, after a newer one launches.

So like a year? or so.... maybe less.
I wouldn't expect all new games, from nintendo to make it onto the older platform, once they launch a new one.
It'll mostly be 3rd party games.

Depends when and if there will be a Switch2 Lite.

The Switch Lite model is not the most selling, but still good for a couple of million per year and is a good family friendly priced model (also) for the multi Switch owning family. Especially if Switch2 will be $400.

If there will be no Switch2 Lite (soon), I can see the Switch Lite will keep on selling for another 4 years. In terms of content there is a mountain of 3DS games to port like Luigi Mansion 2HD.

You just made me think more about likely end-of-life Switch 1st-party games are.

After Switch launched, 3DS got a lot of budget titles. The last first-party game released on 3DS was Kirby's Extra Epic Yarn on 8 March 2019. There were other ports and even some cheap remakes in there.

Some 3DS games that could still come to Switch

-Tomodachi Life

-A Link Between Worlds

-Fire Emblem games (particularly Awakening)

Kid Icarus Uprising would almost surely be a remake whether it comes out for Switch or Switch 2. They'd have to tweak the controls a lot, unless they can do that while keeping the game mostly the same and just upscaling the graphics. 

Until Q4 2025

Switch 2 will be $400 (or even $450 for the card version). That's a too high price for most in the current difficult economic times. Nintendo needs a budget console (even if economics improve), so I guess the Switch 1 will be shipped for about 5 years after the release of Switch 2. In terms of software support, I guess Nintendo will throw "bones" to Switch 1 for about 2 to 2.5 years. However, 3rd parties and especially indies will support it for longer.

However, I strongly think that Nintendo will give the Switch 2 the Game Boy Color treatement: They will market it as a brand new console (i.e. a new generation), however, internally they will count its sales to the current Switch generation. Of course, Switch 2 will be much more powerful compared to Switch 1 than Game Boy Color was to the original Game Boy but still, Nintendo will count it to the same generation. I think Switch 3 will then be a continuation of Switch 2, however, this time around there will be enough innovation for Nintendo to name it a new generation and will not ad the sales to the first generation Switch.

If Nintendo does count the NG towards Switch sales, like the Game Boy Color was in regards to Game Boy sales, then the Switch/NG will be selling near 220M units. That would be a rather ballsy move.

Expect Switch games to be mentioned in Nintendo Directs in both 2025 and 2026 at a minimum. 

They may support it even longer than that though, depending on how they plan to market Nintendo Online.  One strategy would be to gradually use bundles and price cuts for the Switch to get more people to sign up for Nintendo Online.  That would make it easier for people to sign up for this subscription service.  Not sure if they will take this approach or not though.

At any rate, don't expect the Switch to go away any time soon.  Successful consoles hardly ever go away quickly.  Sony tried to kill the PS4 in 2020 and yet new PS4s are still being manufactured and sold.  The Switch will be even harder to kill than the PS4, and that is assuming Nintendo wants to end it's life prematurely (which they probably don't).

↓ They do, they said they want to end it in March 31 2025.