The Collaborative International Dictionary of English v.0.48:

Transience \Tran"sience\, Transiency \Tran"sien*cy\, n.
   The quality of being transient; transientness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

transience
    n 1: an impermanence that suggests the inevitability of ending
         or dying [syn: transience, transiency,
         transitoriness]
    2: the attribute of being brief or fleeting [syn: brevity,
       briefness, transience]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "transience":
   adaptability, adjustability, alterability, brevity, briefness,
   changeability, changeableness, changefulness, compactness,
   compendiousness, conciseness, curtness, death rate, death toll,
   flexibility, fluidity, impermanence, instantaneousness, littleness,
   lowness, malleability, mobility, modifiability, mortality,
   mortalness, movability, mutability, nonuniformity, permutability,
   plasticity, resilience, rubberiness, short time, shortness,
   succinctness, summariness, suppleness, transitoriness