The Collaborative International Dictionary of English v.0.48:

Duration \Du*ra"tion\, n. [OF. duration. See Dure.]
   The state or quality of lasting; continuance in time; the
   portion of time during which anything exists.
   [1913 Webster]

         It was proposed that the duration of Parliament should
         be limited.                              --Macaulay.
   [1913 Webster]

         Soon shall have passed our own human duration. --D.
                                                  Webster.
   [1913 Webster]

WordNet (r) 3.0 (2006):

duration
    n 1: the period of time during which something continues [syn:
         duration, continuance]
    2: the property of enduring or continuing in time [syn:
       duration, continuance]
    3: continuance in time; "the ceremony was of short duration";
       "he complained about the length of time required" [syn:
       duration, length]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "duration":
   abidingness, age, antiquity, changelessness, chronology, constancy,
   continuance, continuation, continuity, defeat of time,
   defiance of time, diuturnity, durability, durableness, duree,
   endurance, firmness, fixedness, frozenness, hardening, immobility,
   immovability, immovableness, immutability, invariability,
   invariableness, inveteracy, lastingness, long standing,
   long-lastingness, long-livedness, longevity, maintenance,
   perdurability, perennation, period, permanence, permanency,
   perpetualness, perpetuity, persistence, persistency,
   psychological time, quiescence, rigidity, run, solidity, space,
   space-time, span, spell, stability, standing, stasis,
   steadfastness, survival, survivance, tense, tenure, term,
   the future, the past, the present, tide, time, timebinding, torpor,
   unchangeability, unchangingness, while