Period of validity of the ESTA
ESTA authorization is valid for two years from the date of authorization. This means that travelers who have obtained it can plan to make several trips to the United States during this period: they will not have to reapply for the document each time they travel. However, you must make sure that your ESTA is valid if you are unsure of when it was obtained.
While valid for multiple trips, an ESTA authorization is strictly associated with a valid passport number. If your passport expires within two years, your ESTA form is no longer valid and will need to be reapplied for (or renewed).
Travelers who have changed their name, gender, or nationality during these two years must also apply for a new ESTA because their personal information is no longer current and therefore no longer valid (an update is not possible). A new application is also required if answers to personal questions (health, criminal history, behavior) have changed.
Please note: an ESTA authorization is valid for two years but does not authorize you to stay in the United States for 24 consecutive months. You can stay in the USA for up to 90 days. For any stay longer than 90 days, a visa or a Green card will be required. Also, if your ESTA is pending, you cannot enter the United States.
Good to know: if your ESTA form expires during your stay in the United States, you will be able to end your stay in peace and will have no problem leaving the country.