Source : Webster's Revised Unabridged Dictionary (1913)

Oftenness \Of"ten*ness\, n.
   Frequency. --Hooker.

Source : WordNet®

oftenness
     n : the number of occurrences within a given time period
         (usually 1 second); "the frequency of modulation was 40
         cycles per second" [syn: {frequency}, {frequence}]