Normally, the pain will start reducing significantly after the first two days if you had surgery. The pain will depend on the problem though and the type of treatment. I think you should consult your doctor he or she will know whether you are getting better and if the progress of recovery if to slow or not.
The pain can last 6 months to a year depending upon age. The bones are cut above and below the knee, so the bones need to heal and regenerate around the prosthesis. It's a throbbing, stabbing, growing type of pain. It helped to have my leg lightly rubbed. I would get up in the night and walk the floor because of this pain. You can get around on a cane after two weeks, but working your knee at home, besides the phyical therapy hurts, and then it still has to heal and regenerate. By 4 months i felt more human; by 6 mo the the pain ended, but feels like half a bowling bowl in your leg all the time, and you can't kneel on the floor or any hard surface ever once it's done. The only up side is that you won't have that excruciating knee pain and at least can function.