U.S. Customs and Border Protection (CBP) have advised that changes in ESTA application processing mean real-time approvals will no longer be available.
International travellers who are eligible for the Visa Waiver Program and plan on using ESTA for travel to the United States should apply as soon as possible but not less than 72 hours before their international flight is scheduled to depart.
It is the passenger’s responsibility to make their application and ensure they have a valid travel authority before they travel. This will make the check-in process much easier for them. Those nationalities not eligible to use the Visa Waiver Program will need to process a visa before travel.
For more information please visit https://esta.cbp.dhs.gov/esta/