Some doctors will advise surgery to all patients with a torn knee ligament. Repairing the damage early is thought to prevent problems later on. An unstable knee can lead to reinjury with more damage. Without a healthy PCL the joint can be too loose and unstable.
Uneven wear and tear on the joint can also result in early arthritic changes in the joint.
Many doctors suggest surgery for patients who have pain, instability, and loss of function. If you can't do what you want to do or if you want to return to your preinjury level of activity, then repairing the ligament is necessary.
It's not clear if there is a point at which you've waited too long. Studies report the average length of time from injury to operation is about one year. Some patients have the operation within three weeks of the original injury. Others have waited up to four years