The Collaborative International Dictionary of English v.0.48:

Inhabitance \In*hab"it*ance\, Inhabitancy \In*hab"it*an*cy\, n.
   1. The act of inhabiting, or the state of being inhabited;
      the condition of an inhabitant; residence; occupancy.
      [1913 Webster]

            Ruins yet resting in the wild moors testify a former
            inhabitance.                          --Carew.
      [1913 Webster]

   2. (Law) The state of having legal right to claim the
      privileges of a recognized inhabitant; especially, the
      right to support in case of poverty, acquired by residence
      in a town; habitancy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

inhabitancy
    n 1: the act of dwelling in or living permanently in a place
         (said of both animals and men); "he studied the creation
         and inhabitation and demise of the colony" [syn:
         inhabitancy, inhabitation, habitation]