After six weeks of resting and treating the knee, a moderate sprain has recovered. A serious strain or sprain might take three to four months to heal. The length of your rehabilitation will be determined by the treatment plan recommended by your doctor and physical therapist, as well as the type of your injury. They will advise when it is safe to return to sports activities. This time frame assumes that you are not doing any further damage to the knee by overstressing it during its recovery. If you continue to play through pain, you can prolong the healing process. Also, if you have another injury, such as a hamstring strain, then that muscle group may need more time to recover than normal due to the amount of work it has to do while your knee heals. Many people who suffer from knee injuries never regain full strength in the knee and remain vulnerable to further trauma. People who play professional sports risk suffering long-term problems with their joints because they endure extensive contact practice and season-long confinement to hotels and training facilities. In conclusion, a knee strain can take up to four months to heal completely. However, if you continue to stress the injured area, it could take longer. It all depends on the severity of the injury and how you manage it during its recovery process.