The half life of a medication is used to decide the dosing time and schedule. There is no relation with half life and the efficacy of the medication. So even after 6 hours the medicine will be equally effective. Also quetiapine comes in an extended release formula also. If your doctor feels, that a longer duration of action is required, that will be practiced to you. Don't worry, the dosing and efficacy are not related.
Next Steps
Trust your doctor. He/she will write a prescription best suited for you.