About 3 months
When you say "knee", that could be many types of fractures.
If it is a tibial plateau fracture
(a fracture of the top part of the tibia), those fractures generally heal in about three months. The healing time can be affected by the health of the patient (smokers and diabetics tend to heal more slowly). Also, the severity of the fracture can impact the healing time. 10-12 weeks is a good estimate.