Google Analytics support page does not provide specifics on how new vs returning users are defined. https://support.google.com/analytics/answer/1012034?hl=en#Behavior
ga.js and analytics.js user tracking. The first time a user visits your site, a _ga cookie is set to help distinguish the user. The cookie has a 2 year expiration date from the time it is set.
If the user visits your site within the 2 year period, they will be marked as a returning visitor. On the return visit, the expiration date for the cookie will be updated to be 2 years from the latest visit.
User clears cookies. If a user clears their cookies, they will be marked as a new user on their next visit.