Almost never. Firstly, many of us have a considerable amount of impacted faeces in our colon. This is hardened, rubbery or wallpaper like material. Substantial work must be done to remove it. Secondly, there is a subtle learning process involved in receiving colonics.
As you become more aware of what is going on in your abdomen, and as 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 may wish to have will depend upon your personal objectives.