How should i treat knee pain (i believe tendonitis) from running and how long does it take to heal?

Rest, ice anti-inflm: Tendinitis or sprain can take 1-2 weeks. Would recommend rest, ice, and anti-inflammatories such as alleve, naprosyn (naproxen) etc (with food). The the knee, i would also recommend quadriceps strengthening exercises - straight leg raises lying flat on the back.

Stretching: Runners usually get tendinitis of the it band on the outside of your knee. Verify this with your doctor. If it is, then stretching the it band is best treatment. Therapy will help. Make sure your shoes are good as well as arch support.

IF it is tendon...: Figure out why you're getting the knee pain. Certain things predispose you to tendonopathy- training errors, foot mechanics, pelvic/hip weakness and rotation. You can alter footwear and the way you run. You can try orthotics and improving foot/ankle mechanics with stretching. Improve strength at hips. Returning to run gradually. Can take weeks to months. Don't run through more than mild pain.