If left constantly running, Outlook in cached mode automatically updates the offline address book on the client every 24 hours. The 24-hour time period is measured from the time that the offline address book was last downloaded successfully. For example, if you complete an offline address book download at 09:00 today, Outlook will start the offline address book download the next day at approximately 09:00. Therefore, different people will receive updates at different, random times.Note The default setting on the Exchange computer is to generate an offline address book differential file every morning at 04:00. For a change that is made in Active Directory to reach the client computer, the following events must occur:
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The change must be picked up by the Exchange computer that generates the offline address book files. This can take several hours. At worst, it can take 24 hours. This variable will be referred to as “x.”
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The Outlook clients must download the offline address book updates every 24 hours. This update can take several hours. At worst, it can take 24 hours. This variable will be referred to as “y.”
The update reaches the client machines x+y hours later. It would be rare for a client to ever experience a 48-hour delay or more unless there were some Active Directory or public folder replication issues.
More Information : http://support.microsoft.com/kb/841273