I remember (not from this site) that one drop per day is the recommended BEGIN dosage. Usually because it is too strong, if pure. Otherwise I would think as many drops that one can ingest is good.
There are some that say oregano should be stopped due to it being anti bacterial and anti viral. As you would not consume a prescribed antibiotic indefinitely. I personally would say, due to it being a natural form, it is not necessary to discontinue. I do not believe one becomes immune to it, in my opinion.