Casts commonly on for 6-8/52, but really depends on the type of fracture, location, healing that goes on etc... Doctor should tell you how long the plaster will be on for...this can change though, sometimes on longer than initially thought due to healing process and fractures stability.
Spiral fractures can be problematic as they can slip and overlap, thus often full leg cast for longer than you feel may be necessary. A full cast stops the knee moving, which in turn limits the amount the tibia moves even more....thus protecting the fracture. Generally the doctors will re-xray, assess how well it is healing and may then reduce the cast. The main thing is to listen to the doctor in terms of weightbearing.
Once they are happy with the healing of the break and the plaster is off and full weightbearing is happening, then build up of activity can begin. Be guided by a therapist though...you don't want to run too quick, when you may not have full movement, strength or balance back. You don't want to develope commensations and bad habits if you can help it.
Sorry can not be more specific...be guided by your specialist and therapists. Ask them the questions and hopefully they will have the answers for you.