The magnetic properties of a disk “fade” over time, the signals become weaker and reading them becomes more difficult. As time goes on, reading becomes hit-and-miss, much later than that and the data cannot be recovered (and likely only partially) without extremely expensive hardware and significant analysis. Eventually the data is completely lost.
Bitrot can be caused by both natural environmental effects, or simply the magnetic corrosion of the media. Estimates put the life of floppy disks from anywhere between 10 and 30 years. Our practical findings show that disks from 1985 are frequently found “rotten”, and thus we would estimate around the 20-year mark is about right for 3.5” DD floppy disks.