Mac OS X 10.8 Mountain Lion removed the ability to set the frequency of checks for software updates. The default frequency is 7 days, use following command to change this:
sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate.plist ScheduleFrequency 1
You’ll need admin privileges to edit this, change ‘1’ in the command to the number of days you prefer. For once in the two weeks you can use 14.
Revert to the default setting:
sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate.plist ScheduleFrequency 7