I don't usually like to cast for more than 6-8 weeks at any one time. Any gains you can make will make any surgery easier and likely more successful. If you can't fix it, careful posting up inside an appropriate orthosis can protect the foot, let it weight-bear in a stable position and protect proximal joints. She is currently undergoing serial casting. Deformities of the foot and ankle are conventionally described accordging to the position of the foot.
Adduction is treated first until the navicular bone is in front of the talus.
Inversion is corrected next by placing the foot in eversion. Equinus is treated last by increasing DF of the ankle. Simultaneous correction may lead to rocker bottom foot. When tx is began within the first weeks after brith the deformity will be completely corrected by non-operative procedures in a relatively short time. I store these cases away, so that when I write my american exams some day, this info may be useful.