Welcome! My name isXXXXX am a UC Davis graduate, and currently, a Medical Director of a veterinary hospital.
I am sorry to hear about this concern about the allergies.
First, if she is doing good, I would be really hesitant to make any changes.
The ol sayin: "if it ain't broke, don't fix it"
But, if you really want to change, symptoms can manifest anywhere from 1 day to a few weeks on the new food.
Also, most over the counter foods are not suitable limited ingredient diets. Why?
Well, the main protein may say fish. But, if there is any other type of protein in there, like some chicken
meal, beef meal, etc..that ruins the diet.
Also, most OTC foods are not processed in "clean" facilities. Meaning, a lot of testing has been done on these OTC foods and they find other proteins in the food all of the time.
Just thought you would be curious about that extra information.
But, the symptoms can manifest quite quickly, but not always.
Please let me know if there is anything I did not cover for you. I hope that information has been helpful.
Please remember to select Reply to Expert, if for any reason you need further clarification, have more questions, or were expecting a different type of answer. DO NOT rate yet if have any other questions. My goal is to try and provide you the best answer possible.