Trust your doctor and pharmacist. It should state right on the bottle how to take your medication, stick with those instructions. Some anxiety medications are made to take on an "as needed" basis, while others need to be taken on a regular basis. Take care!
If your mom is a doctor, and specializes in this area, then run it by the doctor who prescribed the medication. If your pharmacist says take it every 6 hours, I would believe this over your mom if she is not a medical person. If the pharmacist's instructions are contrary to your doctor's instructions, call your doctor for clarification.