Recovery time will depend on the procedure performed.
If a partially torn meniscus was removed - You should be back to full activities within four to six weeks.
If a partially torn meniscus was repaired - You should expect four to six months until full recovery.
If you had an ACL reconstruction - You should commit to an aggressive rehabilitation program and expect full recovery in six to nine months (but you can be back at a desk job within a week or two).
If you had loose bodies removed - Recovery may take two to four weeks.
If you had a loose body repaired - Recovery is usually six to eight weeks.
If you had chondromalacia or arthritis - You should expect a couple of weeks to recover.
You will be encouraged to undertake a moderate exercise routine well before your knee is fully recovered. Generally, you will be discouraged from activities that place excess weight or stress on the knee until full recovery. Non-weight-bearing aerobic exercises, like cycling and swimming, are strongly encouraged during the recovery period. Pool exercises are very helpful.