Time Machine is set to automatically back up every hour, but if you would like to change it to every half hour, you can use the following Terminal (/Applications/Utilities) command:
sudo defaults write /System/Library/LaunchDaemons/com.apple.backupd-auto StartInterval -int 1800
You will need to authenticate as an administrator, since this command is run under a "sudo." The time interval is measured in seconds, so you can enter any time you wish there; just make sure it is in seconds. By default, Time Machine backs up every 3600 seconds (every hour). If you wish to revert to the original, just replace "1800" with "3600."
Want more tips and tricks like this? Visit TUAW's Terminal Tips section!