How Long Would It Take To See Results When Switching To A Better Food If A Dog Had Food Allergies?

My moms great dane has had loose stools recently, has always had a ton of eye “buggers” and red ears (not ear infections but very red and raw looking). I have been telling her to try switching from kibbles n bits to a better quality food and today she switched to Canidae ALS. I know that if these things were food related they would not immediatly correct themselves but what is a reasonable amount of time that if they were food related they should correct themselves?
I personally noticed a reduction in the eye buggers when I changed food last year but is this something food related or could I have just changed when the seasons changed?
Thank you.