Anywhere from a couple of days to 6 weeks.
First off, regardless of whether your dog has food allergies or is just sick – If your dog is experiencing diarrhea or vomiting for more than 24 hours, get him to the vet to make sure he does not get dehydrated.
ALSO, if you feel at any time that his life is in jeopardy – get him to the vet. Sometimes we don’t want to seem over-the-top with our pets, but trust your gut feeling (see side note at the bottom of this post).
Dog food allergies are something that most people are concerned about when they start seeing their dog exhibit excessive scratching, recurring ear infections, or frequent digestive problems, such as diarrhea or vomiting.
However, what normally happens is that you start to feed a new food and over the next week or two, you notice your dog is having one of the problems mentioned above. Then you try to think back, ‘Was he itching that much before and I just never noticed?’ ‘How long have his ears been that bad?’ Chances are, you would have noticed. The thing is, most of the allergy symptoms come on kind of gradual and build up. They itch and scratch a little, then a little more, then it seems like all the time. By the time they have been eating the offending ingredient for 2 to 6 weeks, they will probably be showing full blown symptoms.
And the same thing will happen when you take the offending ingredient away. The allergy symptom won’t stop overnight. It will take a few weeks. You will wonder if there is any difference. For example, when using the Elimination diet to determine a food allergy, they recommend 6 to 8 weeks per ingredient to determine if a food item is an allergic trigger or not, because like the build up when ingesting the food, it is also a ramp down when the food is taken away.
And here is something else to remember: it doesn’t have to be a new food causing the problem. It can be from a food he has eaten for 6 months or 6 years. Just like humans, allergies can develop at any time, even to something we are accustomed to already.
If you believe your dog is suffering from dog food allergies, switch to another dog food (preferably a hypoallergenic dog food as it is one of the healthiest dog food and has limited ingredients) with completely different ingredients than what he is eating. Some people say to make the switch gradually, but that is completely up to you. I switch my dogs foods (very high quality) up all the time and they have no problems – personally, I think they are healthier from changing their diet regularly.
If your dog is just starting to experience problems, it shouldn’t take too long to see a difference after you switch.
If your dog already has full blown dog food allergy symptoms, the change in food could take as long as 6 weeks to see the allergies go completely away.
Side Note regarding your ‘gut feeling’:
I had a dog who suddenly started whining during a walk, so I initially just took him home to rest, but my gut feeling kept telling me to take him to the vet. I felt silly taking him in, like an over-protective mom, since nothing even appeared to be wrong.
Finally, after about an hour of going back and forth, I took him in, and after an x-ray, found that he had swallowed a large glass marble that he could not pass and was blocking his colon. If I hadn’t of taken him in, he would have died in about 2 days! Luckily, I took him in early enough that they did not have to remove much of his intestine (or colon – I don’t know exactly where it was stuck).
Bottom line – trust your gut feeling – if you think there is something wrong with your pet, take them in to the vet!