An interesting item on the MS-DOS floppy disk cache expiry time:
MS-DOS 2.0 contained a disk read cache, but not a disk write cache. Disk read caches are important because they avoid having to re-read data from the disk. And you can invalidate the read cache when the volume is unmounted.
But wait, you don’t unmount floppy drives. You just take them out.
To avoid having to abandon the cache entire, the MS-DOS developers did some benchmarking: How fast can a human being swap floppies in an IBM PC floppy drive?
They couldn’t do it under two seconds.
So the MS-DOS cache validity was set to two seconds. If two disk accesses occurred within two seconds of each other, the second one would assume that the cached values were still good.