How long does it take for all the soft tissue to heal after surgery to repair broken tibia plateau?

Depends. This depends on many factors, including the severity of the fracture, the extent and type of surgery that was done, as well as any co-existing medical conditions such as diabetes, smoking, etc. For a healthy patient with no other medical problems, the soft tissues should largely heal within a couple of weeks. Your doctor will likely follow you closely to keep an eye on your healing.