ESTA유효기간이 있을 겁니다. 아마 2년이지 싶은데요. 만약에 여권유효기간이 2년 보다 짧으면, 여권만료일 까지가 유효합니다.
Your ESTA authorization is generally valid for multiple trips over a period of two years (starting the date that you are approved) or until your passport expires, whichever comes first. As long as you received an ESTA authorization to travel, you do not have to reapply during the validity period.