The FDA stance seems to be summarised here:
Is it legal for me to personally import drugs?
In most circumstances, it is illegal for individuals to import drugs into the United States for personal use. This is because drugs from other countries that are available for purchase by individuals often have not been approved by FDA for use and sale in the United States. For example, if a drug is approved by Health Canada (FDA’s counterpart in Canada) but has not been approved by FDA, it is an unapproved drug in the United States and, therefore, illegal to import. FDA cannot ensure the safety and effectiveness of drugs that it has not approved.
FDA, however, has a policy explaining that it typically does not object to personal imports of drugs that FDA has not approved under certain circumstances, including the following situation:
The drug is for use for a serious condition for which effective treatment is not available in the United States;
There is no commercialization or promotion of the drug to U.S. residents;
The drug is considered not to represent an unreasonable risk;
The individual importing the drug verifies in writing that it is for his or her own use, and provides contact information for the doctor providing treatment or shows the product is for the continuation of treatment begun in a foreign country; and
Generally, not more than a 3-month supply of the drug is imported.
That sentence is an excellent example of imprecision - giving no idea about how it is applied, what it really means, etc. Nonetheless, it does state "-3-month supply".