The processing time for a U.S. ESTA is generally very fast, usually taking from a few minutes to a few hours. However, depending on the details of the application, it may take up to 72 hours in some cases.
Most applicants can check their results immediately after submission or within the same day. However, if additional security screening by the U.S. Department of Homeland Security (DHS) is required, the processing time may be longer.
In particular, applications that include passport information errors, previous U.S. entry records, dual nationality, or travel history to certain countries may not be automatically approved and can experience delays.
For this reason, once your travel to the United States is confirmed, it is safest to apply for ESTA at least 72 hours before departure, and preferably immediately after booking your flight.