Also see:
http://www.microsoft.com/windows/downloads/windowsupdate/faq.mspx
<quote>
How often should I check for updates?
That depends. If you turn on automatic updating, you don't need to
remember to check for Important and Recommended updates for Windows
Vista, or High-priority updates for Windows XP. Windows will check for
new updates whenever you're online and install them using your
automatic updating settings.
If you do not use automatic updating, you should check for updates at
least once each week. Microsoft generally releases updates that cause a
restart on the second or fourth Tuesday of the month. However, some
updates may be released at any time in order to offer you the choice to
install them and realize the benefits as quickly as possible.
For Optional updates for both Windows Vista and Windows XP, you'll need
to go to the Windows Update control panel and select which downloads
you'd like to install.
</quote>
I would like to highlight this phrase:
"If you do not use automatic updating, you should check for updates at
least once each week. Microsoft generally releases updates that cause a
restart on the second or fourth Tuesday of the month."