Yes, if you have not accepted your invitation to Wizened Wizards within 30 days of the approval date, your invitation will be canceled and you may need to reapply (if it has been a long time), or simply send Willowdreamer a private message.
If you don't accept your invitation within a timely manner, we have to assume that you are no longer interested in joining the group - although if that is the case, it would be appreciated if you let us know via a PM before we invite you. If the problem is that you can't find your invitation, we have a brief visual tutorial about that here.
With regard to expiration of membership - we do, periodically, remove members who have never been active over a protracted period of time, generally more than 12 months without posting anything anywhere on Central. Please see the Lurking/Inactivity section in our Code of Conduct for more information.