My right knee is feeling weaker, cracking when knelt on and pain. how long do i wait to see a doctor? (19 year old runner)

Likely runner's knee: If you have not had any injury, but started experiencing anterior knee pain with running, kneeling, or sitting for prolonged periods of time, then you probably have runner's knee. This is pain behind the kneecap from imperfect movement of the kneecap with repetitive flexion/extension of the knee. You could first try exercises for "patellofemoral pain syndrome".

Don't wait: You most likely have at least patellofemoral syndrome. Cartilage wear called chondromalacia and maltracking of the kneecap may also be present. See a sports med specialist who can do a thorough exam to make the diagnosis, can give a specific pt protocol to follow (should include core/hip girdle/postural strengthening+endurance), and can follow your progress/course of treatment. Good luck!