Why is my knee not getting better and how long should I rest it before rehab?

Impossible to know. Without more detail, it's not possible to tell you why your knee is not improving. Knee conditions arise from a variety of reasons, and we would need an understanding of the cause of your knee issue, an examination of your knee, and possibly an x-ray. It is best to make an appointment with your primary care provider for further evaluation, treatment or referral to a specialist.
Depends. There is not enough information to provide a useful answer. If you are having a knee pain that is not getting better you should see your primary care doctor who can then decide on the appropriate next step for you in helping it get better.