Cameron said it took about a year to get the facial rigs build to the point where they would capture perfectly.Dabookerman said:They would have made Maquettes and then scanned them, like Gollum and pretty much every movie CG character out there.
Sculpting them in 3D (zbrush) is reserved for games.
Each character would have have had weeks just rigging the face, so I've no idea how they managed to do so much in Avatar.