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]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "incessancy":
   ceaselessness, chattering, coeternity, consecutiveness, constancy,
   constant flow, continualness, continuance, continuity,
   continuousness, durability, endlessness, equilibrium, eternalness,
   eternity, ever-duringness, everlastingness, everness,
   featurelessness, foreverness, fullness, gaplessness, immutability,
   indestructibility, infinite duration, infinity, interminability,
   jointlessness, monotony, never-endingness, noninterruption,
   oscillation, perdurability, perenniality, perennialness,
   permanence, perpetualness, perpetuity, plenitude, pulsation,
   quick fire, rapid fire, rapid recurrence, rapid succession,
   rapidity, regularity, repetition, seamlessness, sempiternity,
   smoothness, stability, staccato, steadiness, steady state,
   stuttering, successiveness, sustainment, tattoo, timelessness,
   unbrokenness, unceasingness, undifferentiation, uniformity,
   unintermission, unintermittedness, uninterruptedness,
   uninterruption, unrelievedness, vibration