MCL injuries usually heal on their own, but it really depends on where the MCL is torn and how badly it is torn. Many injuries are low grade and respond well to physical therapy and immediate range of motion.
For higher grade injuries, the knee should be braced in a hinged knee brace and recovery can take 3-4 months. Rarely, the MCL is torn off of the shin bone (tibia) and these injuries require repair or in some cases reconstruction with a hamstring tendon from either the same knee or a donor tendon.
In summary, all MCLs are not the same — and in most cases an MRI can predict time to full recovery.
Read about Marvin Bagley III’s injury.