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.