Like TNGlen said, recovery time depends completely on the type of surgery to fix it. Surgical procedure depends on the type of tear in the meniscus (location, depth of it, etc).
They can do a menisectomy, in which they remove the torn portion, trim everything up nice and neat, and can go back to sport in around 6 weeks usually. The down side of this surgery is the long term effects. The meniscus is the cartlidge inside the knee that gives cushion between the femur (thigh bone) and tibia (shin bone). Naturally, the less cartlidge thats there, the less cushion you have, and whats left usually wears out faster over the years.
The other option is a meniscal repair, in which they sow up the torn portion of the meniscus and it has to have time to heal back together before beginning weight bearing activities. A patient with this sugery may just be starting to walk on thier own at this time, and can be up to 3 months before returning to sport. The thing is the blood supply in cartlidge is fairly poor, so it takes a long time to heal.
A smaller tear on the outer ring of the cartlidge can just heal on its own, but the person has to stop doing the activities that bother long enough to allow it to heal up.