Looking through some old stuff today, I found my old music player, a Panasonic SV-SD50. I don't exactly know how long it was sitting idle, but I think it was at least 20 years. It had in it, one of the biggest capacity SD cards I could afford at the time, 256 MB.
I've always been curious about just how long an SD card will hold data; I'd heard 10 years mentioned several times, but I'm sure that was mostly a guess. Anyhow, I put an AAA battery into the player, and all the music was still there, intact. I couldn't say if such longevity applies to more modern SD and micro-SD cards, but they did use larger dies back then, so I couldn't say for sure. But I was surprised at how long this one retained data.