I agree with the others, a second opinion shounds like a great idea. Most ear mite meds will kill the mites in about 10 days, with some of them you may need to repeat the dose again 10 days later for another 10 days but no matter what type of meds you use (as long as they are decent meds from a vet not pet store bought) you should be seeing a big improvement within just a few days.

Are you cleaning the ears too and not just treating them? That needs to be done as well.

Let us know how the other vet works out. Good luck!