Re: A writeable CD left on window sill
An optical disk properly stored away from sunlight in a room with a low level of humidity should logically outlast tape and HDD, probably by a few decades.
My first DVD backup disk was done in 2005, it is still perfectly readable. I don't actually have the date of my first CD backup, but I'm pretty sure it was done around 2000-ish, and it is also perfectly readable. Music CDs I bought in the 90s are also still perfectly readable.
Optical storage is the best thing for personal use. Obviously, I've migrated to BluRay discs now, 25GB size. I'm confident that they'll all be good until the day I die, and then some.
Of course, to be of any use, you have to make the backups first. Joe Public has some issues with that.