If a user does not open their Verification Email within 24 hours, the link will expire. Therefore, the user cannot verify their account and must create a new email and a new account or start the lengthy process of having an admin delete and re-add their account.
However, there is a solution!
If a user’s verification link expires, a new link can be sent via the reset password at the login screen. Once the user follows this link to reset the password, the user's account will verify itself.