How fast does it take for food to travel through the digestive system?
First of all, let's reign in the question a bit and consider only the vertebrate world. Because that's where most of the research seems to focus.
Secondly, let's define what we're talking about: gut transit rate. A specific species' rate will vary depending on environment and diet, as well as whether you're talking liquid or solid waste. So we'll be dealing with general numbers here.
For humans, total digestion time takes anywhere between 24 and 72 hours. According to the Mayo clinic, you're looking at a 53-hour average.
As far as animals go, let's turn to "Comparative Physiology of the Vertebrate Digestive System" by C. Edward Stevens, Ian D. Hume. Here's a quick, comparative reference guide of gut transit times for animals covered in the text:
Cedar waxwing: 0.7 hours Rufous hummingbird: 0.8 hours Elephant shrew: 3.4 hours Ring-necked pheasant: 5 hours Norwegian rat: 13 hours Ostrich: 48 hours Red-eared slider turtle: 58-64 hours Ringtail possum: 112 hours Ringneck snake: 15-35 hours Spectacled caiman: 162 hours Green iguana: 207-386 hours
As you can see, reptiles tend to have the slowest digestion time while birds boast the fastest. Among mammals, the small carnivores win the digestion race and large herbivores finish last.
It's not surprising to see a shrew and a hummingbird at the top of that list as both species are notoriously ravenous to support their super-charged metabolisms. But what are we to make of the Ceder Waxwing (pictured right)? Powered flight requires a high metabolism, but why is this berry-eating flyer beating the hyper-charged hummer?
Here's a little info on the waxwing's gut transit time from a 1991 study published in The American Naturalist:
The hummingbird may boast the higher metabolism rate, but the waxwing has the ability to expedite the gut transit of indigestible seeds.
I primarily tackled this question in "Monster of the Week: The 'Feast' Creatures," but I figured the question deserved specific focus here given the sparsity of answers out there on the various question sites.