Fractured bone generally heal itself. It usually takes 6 to 8 weeks to heal. But the healing time of a fractured bone generally depends on factors such as age of patient, his / her nutritional status and type of fracture etc. Optimal conditions are provided by doctors for fast healing .The only role of plaster of Paris is to support and hold the fractured bone in place during the healing period.
Thus in a given period of time, if provided with proper care, a bone has ability to heal itself.