How long can I wait before having my MCL repair after it has been diagnosed?

Depends. Many mcl tears can be treated with bracing an pt. It depends on the location of the tear. Ask your orthopaedic surgeon.
Indefinitely. Most mcl tears do not require surgery and are treated with a brace and rehab. Grade 1 sprains and grade 2 partial tears do well with nonoperative care. Grade 3 tears are inherently unstable but most of these do well without surgery too. Occasionally there is a persistent sensation of medial joint space opening when stressed in the knock knee position and buckling with activity requiring surgery.