Pain after knee replacement surgery usually peaks within the first few days and gradually decreases over the following weeks and months. Your pain should be significantly improved by six weeks post-surgery and continue to ease with physical therapy and return to a more active life. Pain after knee replacement surgery usually peaks within the first few days and gradually decreases over the following weeks and months. Your pain should be significantly improved by six weeks post-surgery and continue to ease with physical therapy and return to a more active life.