timppu: So, floppy disks? For what? I am pretty sure that if you can buy some old MS-DOS games on floppies, the magnetic data from those floppy disks has perished already decades ago, and the floppies are useless.
PixelBoy: I have several floppy disks, both factory made and own saves, disks for PC and Amiga, 3,5" and 5,25", stuff bought in unopened boxes and pre-owned stuff that has gone through several owners.
To this date, I haven't encountered a disk that wouldn't work.
The magnetic "decay thing" is a myth.
Googling for it, the lifespan of floppy disks seems to be estimated to be around 10 years (or less), at best 20 years. And those estimations are for pretty much perfect storage conditions. Considering the (hard cover) floppy disks (games) were a thing around 1986-1994 or so, ie. about 30 years ago, at least I wouldn't take my chances e.g. buying a floppy disk game on Ebay, and still expecting it to work perfectly.
It has been a long time since I threw all my floppy disks to trash, but back when I tried them, many of them were failing already. I kept them in a relatively cool and dry place, Finnish climate is pretty much perfect when it comes to preservation of magnetic or optical media (relatively dry and cool).
Also I didn't find any information with google that the magnetic decay would be a myth (on floppy disk, magnetic disks and even hard drives). Quite on the contrary, many of the articles said that over time, the magnetic data will get weaker and weaker, and fail at some point. How fast, that depends how they are stored, whether there are any stronger magnetic fields nearby etc.