The Collaborative International Dictionary of English v.0.48:

Incessancy \In*ces"san*cy\, n. [From Incessant.]
   The quality of being incessant; unintermitted continuance;
   unceasingness. --Dr. T. Dwight.
   [1913 Webster]

WordNet (r) 3.0 (2006):

incessancy
    n 1: the quality of something that continues without end or
         interruption [syn: continuousness, ceaselessness,
         incessancy, incessantness]