Due to the stringent regulations and standards that must be adhered to, it may take up to 14 weeks (or 3.5 months) for a Yankee bond to be offered to the public. Part of the process involves having debt-rating agencies evaluate the credit worthiness of the Yankee bond's underlying issuer.
Foreign issuers tend to prefer issuing Yankee bonds when U.S. interest rates are low because this means lower interest payments for the foreign issuer.
Investment dictionary. Academic. 2012.