Source : Webster's Revised Unabridged Dictionary (1913)

Lengthiness \Length"i*ness\, n.
   The state or quality of being lengthy; prolixity.

Source : WordNet®

lengthiness
     n 1: amount or degree or range to which something extends; "the
          wire has an extension of 50 feet" [syn: {extension}, {prolongation}]
     2: the consequence of being lengthened in duration [syn: {prolongation},
         {continuation}, {protraction}]