It may take 45 minutes to complete the configuration if the mailbox server is offline. However, it takes only 15 to 20 seconds to complete the configuration if the mailbox server is online.
When Active Manager makes a remote procedure call (RPC) to the mailbox server to retrieve the status of the database, Active Manager encounters a time-out issue when the Mailbox server is offline. However, because the failure status is not cached, Active Manager continues to make an RPC to retrieve the status for each mailbox. Therefore, if there are many mailboxes in the database, this process can take a very long time.
To resolve this issue, install the update rollup that is described in the following article in the Microsoft Knowledge Base:
2685289 Description of Update Rollup 3 for Exchange Server 2010 Service Pack 2
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about the Set-Mailbox cmdlet, go to the following Microsoft website: