If you provided ESTA with your email when you applied, you will receive an expiration notice when your ESTA is about to expire. The email will advise recipients to go to the official ESTA Web site to reapply. See sample notice below:
ESTA Expiration Warning?: ATTENTION! Your travel authorization submitted on (date) via ESTA will expire within the next 30 days. It is not possible to extend or renew a current ESTA. You will need to reapply at https://www.visa-waiver.com if travel to the United States in intended in the near future. If there are 30 or more days left on the old authorization you will receive a warning message during the application process and be asked if you wish to proceed.
As a general rule, your ESTA approval is good for multiple entries into the U.S. over a two-year period. Exceptions to this validity period include applicants whose passports will expire before the two year validity period is complete - in which case the ESTA approval is only granted until the expiration date of the Passport.