The Collaborative International Dictionary of English v.0.48:

Perishableness \Per"ish*a*ble*ness\, n.
   The quality or state of being perishable; liability to decay
   or destruction. --Locke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

perishableness
    n 1: unsatisfactoriness by virtue of being subject to decay or
         spoilage or destruction [syn: perishability,
         perishableness]