If your running outlook 2003, or have your mailbox on a exchange 2003 mailbox server this is why. With Exchange 2003 and Outlook 2003 there were big delays in free-busy information getting updated:
- By default, Outlook only updated free and busy information every 45 minutes. Furthermore, because of bandwidth and scalability issues, you could not decrease this interval.
- There were latencies that resulted from public folder replication - 15 minutes by default.
- In cross-forest scenarios, there were delays when you used the Microsoft Exchange Inter-Organization Replication tool to replicate free and busy information across forests.
If you move all your users accross to an Exchange 2007/2010 mailbox server, the availability service is used instead of public folders. With the availability service there is a very small delay (60 seconds) for information to be retrieved from outlook clients. This also requires Outlook 2007/2010 to ensure that you use the availability service instead of public folders.