How long should I wait to work out after a knee injury?

Depends on injury. Depending on the severity and treatment requirements it could be anywhere from a week to 6 months before returning to full activity. This is definitely a question for your physician.
2-4 weeks. Your doc will tell you exactly the period - studies have shown that you can maintain your fitness level even if you need to change or cut back on your exercise for several months. In order to do so, you need to exercise at about 70 percent of your vo2 max at least once per week. Even if one body part or joint is immobilized, there's usually no reason that you can't crosstrain to stay fit.

Related Questions

How long after a knee injury before you can work out?

Knee Injury. It depends on the injury and the severity. This is something you need to work out with your doctor in conjunction with a physical therapist as working out will be part of the rehabilitation process of your knee. Read more...
Depends. On the injury. If rice is enough to alleviate your pain, you may return to your desired level of activity slowly. If you pain persists you should see a physician for a diagnosis and treatment plan to follow. Read more...