This was the description i came across.
http://www.sparkpeople.com/resource/health_a-z_detail.asp?AZ=276
Like other types of sprains, knee sprains are classified according to a grading system:
Grade I (mild) — This injury stretches the ligament, which causes microscopic tears in the ligament. These tiny tears don't significantly affect the overall ability of the knee joint to support your weight.
Grade II (moderate) — The ligament is partially torn, and there is some mild to moderate instability (or periodic giving out) of the knee while standing or walking.
Grade III (severe) — The ligament is torn completely or separated at its end from the bone, and the knee is more unstable.
Another website claims
http://www.slideshare.net/jameskzane/sprained-knee-recovery-time
grade 1-few days to a week of down time
grade 2-one week to a month
And grade 3 you can imagine several weeks to several months, i assume he does not have that, as i also read it would likely require surgery.