Quick answer: 20 years from initial patent application.
Explanation / Long answer: This time period is called the "patent term". In the U.S., it starts the day the patent office receives the first application. This starting point is completely regardless of when the patent is approved, when the FDA approves the drug, or when the drug actually hits the market, etc. From that application date, the patent will protect the proprietary formula for 20 years.
Possible Extension: However, if the drug's innovator can prove that a federal agency delayed (beyond reason) the process of getting the drug to market, they can apply for an extension for up to 5 additional years. But, if there is still at least 14 years of patent remaining, it would not be approved. Or, if approved it would be granted for a time period up to 5 years to match up to 14 years of protected marketing.
For the purpose of studying for the Nat'l Exam, knowing the quick answer (above) and that an extension is available is all you need to know.