The Collaborative International Dictionary of English v.0.48:

Lengthy \Length"y\ (-[y^]), a. [Compar. Lengthier
   (-[i^]*[~e]r); superl. Lengthiest.]
   Having length; rather long or too long; prolix; not brief; --
   said chiefly of discourses, writings, and the like. "Lengthy
   periods." --Washington. "Some lengthy additions." --Byron.
   "These would be details too lengthy." --Jefferson. "To cut
   short lengthy explanations." --Trench.
   [1913 Webster] Lenience

WordNet (r) 3.0 (2006):

lengthy
    adj 1: relatively long in duration; tediously protracted; "a
           drawn-out argument"; "an extended discussion"; "a lengthy
           visit from her mother-in-law"; "a prolonged and bitter
           struggle"; "protracted negotiations" [syn: drawn-out,
           extended, lengthy, prolonged, protracted]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "lengthy":
   a mile long, boring, colossal, de longue haleine, dragging,
   drawn-out, dull, elongate, elongated, endless, extended, extensive,
   far-flung, far-reaching, fargoing, filled out, gangling, gangly,
   garrulous, giant, gigantic, interminable, lank, lanky, leggy, long,
   long-drawn-out, long-legged, long-spun, long-winded, longiloquent,
   longish, longsome, loquacious, no end of, overlong, padded, prolix,
   prolonged, protracted, rangy, sesquipedal, sesquipedalian,
   spun-out, statuesque, talkative, tall, tedious, unrelenting,
   verbose, windy, without end, wordy