The Collaborative International Dictionary of English v.0.48:

Oftenness \Of"ten*ness\, n.
   Frequency. --Hooker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

oftenness
    n 1: the number of occurrences within a given time period; "the
         frequency of modulation was 40 cycles per second"; "the
         frequency of his seizures increased as he grew older" [syn:
         frequency, frequence, oftenness]