I’d say 12 hours is a great amount of time to get into either Breath of the Wild (if you haven’t played it yet), or else Tears of the Kingdom (if you’ve played BotW).
I got like 50h on BoTW but I never got into it
Ah that’s a shame. What about Mario Odyssey? Another superb game with lots to do.
Otherwise I guess something like the GTA Trilogy, TMNT: Cowabunga Collection.
All depends what kinds of games you like though, I guess.
Hades is my vote. Short runs mean you can take breaks/naps/whatever. Great story telling if you get hooked and want to play for hours.
I would say fire emblem 3 houses.
You need a spare battery. Nintendo switch won’t survive that long.
Final Fantasy Pixel Remasters, BOTW/TOTK, Pokémon Violet or Legends Arceus.
Something like stardew valley , golf story and sports story would suit a plane journey quite well. Although as others have stated I’ve never found the switch Has held my attention for that long and I prefer to just read or try and sleep on long flight.
Best of luck finding something suitable
Edited to say I’ve been enjoying paper Mario 64 on nso and that might be good but I can’t remember if nso can be accessed offline.
I ended up playing Fire Emblem Awakening on the 3DS lol
Witcher 3.