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. If your ESTA expires while in the US it will not affect your departure.
Para maior clareza, isso não afeta seu limite de 90 dias no Programa de Isenção de Visto.