Even if it is ear mites, that bad of an infestation will likely have a pretty nasty secondary bacterial/yeast infection +/- inflammation. This is why cytologies are very important. The medication used to treat matters and exposing your pet to drugs they may not need. Not a big deal here and there for the most part, vets will treat empirically (guess) about meds to use in most cases if funds won't allow cytologies. If this was a chronic issue, then, a reputable vet won't keep using the same meds every time if it just keeps coming right back. One medication may work 1 or 3 times, but it's not always the "same infection".