I'm not a doctor but I've had three knee operations. If it's a minor tear and can be cleaned up with a scope and then let your body do most of the healing of the ligament, you're probably looking at 2-3 months.

If you need a full reconstruction (I've done my ACL twice) you're looking at 6-7 months.

Regardless, you can speed your recovery by keeping it as mobile as possible, working to get your range of motion back as quickly as possible (through physical therapy, ice/heat regularly, etc). And all of this is of course dependent on the individual and the exact nature of the injury. I'm just speaking from experience.