I currently have a 48 hour guest pass network with a 24 hour grace period. At the end of the 48 hour, some clients get the "Email already exists!" error message when they try to create a new guest pass.
In my somewhat limited testing I've found that it takes more than 15 minutes, but less than 60 minutes before the client can create a new guest pass. It seems like there is a delay in the purging of expired guest pass records.