Almost never.  Firstly, many of us have a considerable amount of impacted faeces in our colon; these is hardened, rubbery or wallpaper like material.  Secondly, there is a subtle learning process involved in receiving colonics.  As you become more aware of what is going in your abdomen and your body learns how to allow the cleansing experience you are better able to enter into that process and therefore more material is released.  One colonic will remove some of the stagnant waste in the colon.  The second and subsequent colonics will remove more.  How many you wish to have will depend upon your personal objectives.  Your qualified therapist will advise you after your initial colonic treatment.