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]

The Collaborative International Dictionary of English v.0.48:

Lengthen \Length"en\, v. i.
   To become longer. --Locke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lengthen
    v 1: make longer; "Lengthen this skirt, please" [ant: shorten]
    2: become long or longer; "In Spring, the days lengthen" [ant:
       shorten]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "lengthen":
   add to, aggrandize, amplify, augment, boost, broaden, build,
   build up, continue, dawdle, drag out, draw, draw out, dwell on,
   elongate, enlarge, exalt, expand, extend, fatten, fill out, hike,
   hike up, hold, increase, inflate, jack up, jump up, keep,
   keep alive, keep going, keep up, lengthen out, let out, linger on,
   maintain, maximize, parlay, perpetuate, preserve, procrastinate,
   produce, prolong, prolongate, protract, pull, put up, pyramid,
   raise, retain, spin out, strain, stretch, stretch out, string out,
   sustain, tauten, temporize, tense, thicken, tighten, up