The human colon can hold a tremendous amount of fluid - 4 quarts for a normal sized adult, which is typically what is given in a Barium X-ray study. In excess of this could lead to a colon rupture, requiring emergency surgery, and a long course of antibiotics. For cleansing purposes, a series of enemas is required - generally I recommend 2 quarts for the first two, 4 quarts for the next two, and one or two two-quart final cleansing enemas. To speed up cleansing, add a package of castille soap to the second enema.
I strongly urge anyone practicing enema play to refrain from adding anything other than castille soap, or glycerin to an enema. The blood supply of the colon enters the hepatic portal vein, so anything absorbed here goes directly to your liver. I have heard of many stories in my career of persons adding alcoholic beverages to enemas - it will make you drunk very fast, and could easily lead to alcohol poisoning. Vinegar is another substance that is nasty - Acetic acid is the eventual breakdown product of alcohol, and the liver can be damaged with an overload - excess acetic acid load is what leads to alcoholic cirrhosis. So it is fair to say that any substance placed directly in the colon is far more dangerous than taken by mouth, as it is absorbed extremely rapidly, leading to acute toxicity, as it has nothing to compete with for absorbption.