Mountain Lion removed the ability to set the frequency of checks for software updates. The default frequency was always a week and it remains so.
To change the frequency, open com.apple.SoftwareUpdate.plist, found in /Library/Preferences, and change the ScheduleFrequency key from 7 to the number of days you prefer.
I prefer to be notified of updates daily, if available, so I changed the frequency to 1.
[kirkmc adds: Note that you'll need admin privileges to edit this file. You can do so with a text editor, or with Xcode, which presents a graphical interface for .plist files. ]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120824124203635