It really depends. Somethings I can tell almost immediately, like food intolerance issues, is it causing gas, does my dog act like he has a stomach ache, that sort of thing. Some things take 3-7 days, does it cause ear issues, itching, anal gland issues. Some take weeks, dry skin, body odor, eye staining. I have always know within 3 weeks except once when the dog had so many other things going on that it probably took me 3-4 months before I noticed that I didn’t like running my hands through his coat as much. His coat texture had changed.
