Articulate License Expiration

We have a "bank," if you will, of 100 licenses that are assigned upon request. When we received the licenses we received requests by some who never logged in after the request was sent. At this point I've started receiving notification that some of the users have never accepted their access invitation and I've had one person whose access invitation expired. I am just wondering what the timeframe is around the notification that a request has not been accepted and the actual expiration of the invite. I hope that's clearer! Your post addresses the expiration question perfectly!