The Collaborative International Dictionary of English v.0.48:

Lengthen \Length"en\ (-'n), v. t. [imp. & p. p. Lengthened
   (-'nd); p. pr. & vb. n. Lengthening (-'n*[i^]ng).]
   To extent in length; to make longer in extent or duration;
   as, to lengthen a line or a road; to lengthen life; --
   sometimes followed by out.
   [1913 Webster]

         What if I please to lengthen out his date. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lengthening
    n 1: the act of prolonging something; "there was an indefinite
         prolongation of the peace talks" [syn: prolongation,
         protraction, perpetuation, lengthening]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "lengthening":
   continualness, continuance, continuation, continuity, crescendoing,
   crescent, elongation, endurance, expanding, extension, growing,
   increasing, incremental, intensifying, lingering, maintenance,
   multiplying, on the increase, perpetuation, perseverance,
   persistence, procrastination, production, progress, progression,
   proliferating, prolongation, protraction, pursuance, repetition,
   run, snowballing, spreading, staying power, straight course,
   strain, stretch, stretching, stringing out, sustained action,
   sustenance, swelling, tension, tightening, uninterrupted course,
   unremittingness, waxing, way