You Had Me At EHLO... : Free Busy generation process: "Free Busy generation process
From working with customers there seems to be some confusion around how Free Busy information is generated and read by the client. There is even further confusion about how to balance Free Busy access across multiple sites. In this post I hope to clear most of that up and provide some guidance that will help you prevent issues with Free Busy."
You Had Me At EHLO... : The free/busy data story: "The free/busy data story
Exchange Free Busy (FB) data itself is not simple or easy to read. This will describe what the data looks like in the Exchange store, how it’s calculated and processed through the Outlook client.
Microsoft Exchange: Publish different free/busy time periods: "Microsoft Exchange: Publish different free/busy time periods"
By default, Outlook will publish two months of free/busy information. While you're able to change this interval in Outlook (by going to (Tools > Options > Calendar Options > Free/Busy Options), Outlook Web Access is also subject to this two-month default, which can prove to be quite inconvenient since OWA users don't have an easy way to change this setting. To make matters worse, even if you do configure Outlook with a longer interval, Outlook may eventually revert back to the two month default. While you can run Outlook with the /cleanfreebusy switch to correct the problem, it still may not "stick."

The solution: On your Exchange servers, add a registry key that redefines the two month default value into something more suitable for your organization.

-Open the registry editor.
-Browse to the key HKLM\Software\Microsoft\ExCDO\Parameters.
-Under this key, add a DWORD entry named "FBPublishMonth".
-The value of this new entry is a number, which is the number of months for which you would like to publish free/busy information across the board, for all users.
-Restart IIS and the System Attendant services on your Exchange servers.
Help and Support: "Microsoft Support Lifecycle
Microsoft’s Support Lifecycle policy provides consistent and predictable guidelines for product support availability at the time of product release and throughout the product life. By understanding the product support available, customers are better able to maximize the management of their IT investments and strategically plan for a successful IT future. "