Anyone with Ulcerative Colitis, Crohn's Disease (or any Inflammatory Bowel Disease or disease of the intestines) will have a pretty good idea of how quickly they absorb and pass foods from their system.
For some it seems unnaturally quickly, for others it may be a relatively normal amount of time.
According to research, with results listed by the Mayo Clinic, it takes an average of approximately 8 hours for food to travel through the stomach and small intestine, and then an additional approximately 40 hours to fully travel through the large intestine in healthy persons.
To test this out for yourself, make sure to eat colorless foods for a couple of days prior, and then one day eat some beets. Particularly red beets. (Not golden beets).
Then, when you notice a red discoloration appearing like blood when you go to the washroom, there is your approximate answer.
Now, of course, if you already have an inflammatory bowel disease, it might be difficult to decipher between beets and blood... and if you have allergies or negative effects while eating beets, please do not participate.
For me, with my large intestine having been removed several years ago, I can automatically remove essentially 40 hours from my digestion time.
Though, when I still HAD my large intestine, because it did not function at all properly, it never (since age 14) took longer than a couple of hours to digest food. There were even instances of digestion taking a mere 10 minutes because of the widespread inflammation.
Anyways - nowadays it appears to take an hour, maybe two, on a good day for food to travel through my system.
The Beet Experiment.