Final answer:
The patient should take the medication twice a day, once in the morning and once in the evening.
Step-by-step explanation:
When a prescription says 'bid', it means that the patient should take the medication twice a day. In this case, the patient should take metronidazole 500 mg twice a day for a duration of 7 days. This means they should take the medication once in the morning and once in the evening, with an approximate 12-hour interval between each dose.