The feeling bad part... I mean it only gets really bad for 2 or 3 days I found in my own case. But like VX said detoxing does funny things, it could last longer. But after awhile during the juice fast the wall you hit comes down, and you have more energy, stamina, and well being then you have ever felt your whole life.
As for the results of the juice fast... those will only last as long as you keep a healthy diet.