Thanks for the great question.
Yes, and no. A Member is identified by being logged into your site, via browser cookie authentication. So, if a logged-in Member saves a Post/Page into their favorites... upon return, IF they're still logged in with proper permissions, they will continue to have access; the bookmark WILL work fine
That being said, some Members will NOT check the "remember me" box upon logging in, and so they might have to log back in, before they can view protected areas of your site; even if those Posts/Pages are saved in their favorites. In other words, they MUST always be logged in.
By default, if the "remember me" box is un-checked during login, their login session will last until they close their browser window. So if they log in; and then close their browser window, they'll have to log back in all over again. Checking the "
remember me" box makes it possible for WordPress/s2Member to keep their session alive for up to one year at a time.