Invitations expire, so an old link found in an old email cannot quietly let somebody into your Circle a year later.
The two windows
Family member: 14 days.
Professional caregiver: 7 days. Shorter on purpose, because a caregiver seat carries access to health information.
The clock starts when the invitation is created, not when it is read.
What happens after that
The link stops working and shows a message saying so. Nothing is lost and nothing is broken. Send a new invitation and it works exactly as the first one would have.
See My invitation expired. What do I do?