Hi Deepa,
Recovering from an injury depends upon the injury site, type and severity. For e.g. If its any particular ligament injury it will take a long time and if its a muscle injury it recovers fast again depending on its severity. So its better to resume your workout only after getting a green signal from your doctor. In first week of workout include only some very light aerobic exercises and knee muscles strengthening exercises like Heel press and knee press. Then after 2 nd week start moving towards your normal workout but make sure your workout plan must include stretching and Warm up and Cool down exercises.