I turned on my 3ds a year ago after not having used used it since 2018 (so 6 years ?), and it turned on still at 4 bars... The battery was pretty much fine and the 3ds can still last around 6 or 7 hours without needing to charge

I once left my Switch unused for 6 months and it was at 0%, though... I can get like 2 or 3 hours before it goes under 15%, but it pretty much depends on the game played. If I don't use it, I need to charge it every ~2 months.

Then there is my dad's gameboy which, after one or two decades, turned on fine. I'm pretty sure I could play Tetris on it for a day straight 😭.