Contributed By: Cliff Hobbs [MVP SMS]
Reproduced from the Microsoft Software Update Services: Frequently Asked Questions web page:

http://www.microsoft.com/windowsserversystem/sus/susfaq.mspx#top

"The Windows Update Web site is refreshed with security updates (and other content) at the same time as security bulletins are released. We strive to release updated SUS content at the same time as it becomes available on Windows Update, but occasionally there may be a short delay (2 to 3 hours)."