When Do Registration Links Expire?

Registration links virtually never expire, with some exceptions:

Users receive a reminder 55 days after being added to the Portal, if the haven't registered yet. This reminder email includes a new, unique link. Once a new link is generated/sent, the original one no longer works. 

Additionally, if a facilitator uses the "Resend link" button for an unregistered user, any previous link is no longer valid.