Unless revoked, travel authorizations are valid for 2 years from the date of authorization, or until your passport expires, whichever date comes first. The Authorization Approved screen displays your travel authorization expiration date.
Your ESTA authorization is generally valid for multiple trips over a period of 2 years or until your passport expires, whichever date comes first. Once you received an ESTA authorization, you do not have to reapply during the validity period.
If your ESTA expires while in the U.S. it will not affect your departure. Your ESTA only needs to be valid at the time of entry into the U.S.
Note: Is it important to PRINT a copy of the document for your records.
Receiving ESTA authorization does not mean you may stay in the U.S. for two years. It only allows you to travel to the U.S. under the terms of the Visa Waiver Program (VWP), which only allows you to stay in the U.S. for up to 90 days.