The amount of fecal matter in the bowel, at the time of death, depends on the amount of food eaten, the type of food eaten, the state of the persons digestive system, and the frequency of bowel movements, of the person being examined. There can be some large variations particularly if the person has any sort of bowel obstruction, (partial or complete), such as tumors, twists, or foreign objects.
If your worried about a buildup of old fecal matter, give yourself a week of DE, (one teaspoon in a glass of water), this does a very good job of clearing any pockets of hardened fecal matter from the bowel.