This was answered by Portal Development:
It's possible to change the activation email token expiry but it requires a change to a file that cannot be customized by the customer. We will enable a configuration option in our next CR. Portal 3.5 CR10 is planned for November 2018.
The file responsible for sending the activation email is /sitebuilder/installed/pagerules/ajax.xml. You need to add expiryDays="14" to <AccountApprovalXS> in the <PageRule match="/ajax/(accept|reject)-accounts">. The expiryDays attribute defaults to 7 if missing. This file is overwritten when CRs are applied which is why don't support customer modifications.