Digestion time varies between individuals and between men and women. After you eat, it takes about six to eight hours for food to pass through your stomach and small intestine. Food then enters your large intestine (colon) for further digestion, absorption of water and, finally, elimination of undigested food.
In the 1980s, Mayo Clinic researchers measured digestion time in 21 healthy people. Total transit time, from eating to elimination in stool, averaged 53 hours (although that figure is a little overstated, because the markers used by the researchers passed more slowly through the stomach than actual food). The average transit time through just the large intestine (colon) was 40 hours, with significant difference between men and women: 33 hours for men, 47 hours for women.
Two British doctors studied digestion time in children. They fed 35 children juice containing a red marker and asked the children's mothers to note when the stool first turned red. The mean time of transit from mouth to anus for the group was 33 hours (meaning half the children had digestion times slower than this and half had digestion times greater than this).