Middle C has a frequency of about 261.6 Hz.
Lowering by one octave halves the frequency.
261.6 Hz / 257 ~= 1.815*10-15 Hz
That’s right, 1.815 fHz (femtohertz)
Since Hz is frequency in cycles per second, this is corresponds to a period of about 550899036987216 seconds per cycle, or nearly 17.5 million years.