No person employing another person to sell at retail drugs and medicines or to compound physicians’ prescriptions shall require or permit such employee to perform any work in any store, dispensary, pharmacy, laboratory, or office for more than an average of nine hours per day, or for more than 108 hours in any two consecutive weeks or for more than 12 days in any two consecutive weeks, except that any registered pharmacist may be so employed and may perform such work for the full period of time permitted by this section.