How long will it take to heal a stress fracture under the knee cap?

It depends...: Stress fractures of the patella or the groove in the femur are relatively uncommon. Healing time depends on many factors: severity of the fracture, age of the patient, compliance with rest and immobilization and whether other negative healing factors are present (smoking, diabetes, etc.). Generally speaking, 8-12 weeks would be typical time for healing.

8+++ wks, maybe more: Stress fractures of the patella are rare and diagnosis is often delayed by months. Nonoperative treatment involves immobilization and protected weight bearing (ie stay off it). Nonop tx is often unsuccessful and then surgery is the recommended. Healing takes 2+ months, often 3-4. You need to be followed closely. See a knee specialist soon. Good luck!