They really don't know. It really depends exactly where it is, near one of the ends of the bone or in the middle, as well as the length of the "crack" for lack of an easier way to describe it. The stress reaction essentially isn't healing. The bone callus isn't developing properly so the healing isn't occuring. They put the rod in the bone to hold the callus in place so healing can occur. They may use electrical bone stimulators as well. There really is no way to predict how long it will take as the normal healing process, already isn't occurring. I'm assuming they've already gone through all of the nutrient/mineral deficiencies he may have.