My acoustics prof (David Blackstock) always said "nearfield is when wavefronts are approximately planar, and farfield is when wavefronts are approximately spherical". There is an impedance derivation for farfield (re: Bruno above) pointing to the Rayleigh distance (0.5*ka^2), but I'm not at all clear how room effects come into play. I'd venture a guess that farfield is pretty much not going to happen in the average room, but I'll leave it up to the others to tell me whether that's right or wrong.