Approved ESTA applications are valid for a period of two years, or until the passport expires, whichever comes first, and multiple trips to the United States without the traveler having to re-apply for another ESTA. When traveling to the U.S. with the approved ESTA, you may only stay for up to 90 days at a time - and there should be a reasonable amount of time between visits so that the CBP Officer does not think you are trying to live here. There is no set requirement for how long you must wait between visits.