The magnetic strength of particles on the disk can decay at 1% per year, but the drive won't have issues reading them until they fall below a threshold where they can no longer be read. It could take decades.

Magnetic HDDs also tend to have inbuilt SMART features to monitor disk performance and health, so they can inform when they are beginning to give problems. So the advanced user can recover the data before the HDD fully fails.

In my experience, flash drives tend to get problems suddenly leading to data corruption, and it may not be immediately apparent to the user till it is too late. I haven't such problems in recent years though, so maybe flash drives have become smarter too.