The Collaborative International Dictionary of English v.0.48:

Permanence \Per"ma*nence\, Permanency \Per"ma*nen*cy\, n. [Cf.
   F. permanence.]
   The quality or state of being permanent; continuance in the
   same state or place; duration; fixedness; as, the permanence
   of institutions; the permanence of nature.
   [1913 Webster]

WordNet (r) 3.0 (2006):

permanence
    n 1: the property of being able to exist for an indefinite
         duration [syn: permanence, permanency] [ant:
         impermanence, impermanency]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "permanence":
   abidingness, age, antiquity, application, assiduity, assiduousness,
   bulldog tenacity, ceaselessness, changelessness, coeternity,
   concentration, constancy, continualness, continuance,
   defeat of time, defiance of time, dependability, diligence,
   diuturnity, dogged perseverance, doggedness, durability,
   durableness, duration, endlessness, endurance, engrossment,
   eternalness, eternity, ever-duringness, everlastingness, everness,
   fidelity, fixedness, foreverness, immutability, incessancy,
   incommutability, indefatigability, indefeasibility,
   indestructibility, industriousness, industry, inertness,
   infinite duration, infinity, insistence, insistency,
   interminability, intransmutability, irretrievability,
   irreversibility, irrevocability, lastingness, long standing,
   long-lastingness, long-livedness, longevity, loyalty, maintenance,
   never-endingness, obstinacy, patience, patience of Job,
   perdurability, perennation, perenniality, perennialness,
   permanency, perpetualness, perpetuity, perseverance, persistence,
   persistency, pertinaciousness, pertinacity, plodding, plugging,
   preoccupation, relentlessness, reliability, resolution, sedulity,
   sedulousness, sempiternity, single-mindedness,
   singleness of purpose, slogging, stability, stamina, standing,
   staying power, steadfastness, steadiness, stick-to-itiveness,
   stubbornness, survival, survivance, tenaciousness, tenacity,
   timelessness, tirelessness, unalterability, unceasingness,
   unchangeability, unchangeableness, unmodifiability,
   unremittingness, unrestorableness, unreturnableness,
   unswerving attention