I know and you know that backup is vital but sometimes it is too often! And the mighty Apple, on some applications, especially on Time Machine, doesn't give you many things to configure. Thanks God for the Terminal!

Now, let's assume you know what and where the terminal is and open it (If not, do a spotlight search ;).
You want to see the current backup interval (which is one hour in seconds, meaning 3600):

mbpro:~ florian$ [color=green][b]sudo defaults read /System/Library/LaunchDaemons/com.apple.backupd-auto[/b][/color]
{
    Disabled = 1;
    EnableTransactions = 1;
    KeepAlive = 0;
    Label = "com.apple.backupd-auto";
    ProgramArguments =     (
        "/System/Library/CoreServices/backupd.bundle/Contents/Resources/backupd-helper",
        "-auto"
    );
    RunAtLoad = 0;
    SHAuthorizationRight = "system.preferences";
    [color=red]StartInterval = 3600;[/color]
}

Now, you want to change the value from one hour to four. For this, you type:

mbpro:~ florian$ [color=green][b]sudo defaults write /System/Library/LaunchDaemons/com.apple.backupd-auto StartInterval -int 14400[/b][/color]
Password: [color=yellow](you put here your user's password)[/color]

And voila! You have your time modified:

mbpro:~ florian$ [color=green][b]sudo defaults read /System/Library/LaunchDaemons/com.apple.backupd-auto[/b][/color]
{
    Disabled = 1;
    EnableTransactions = 1;
    KeepAlive = 0;
    Label = "com.apple.backupd-auto";
    ProgramArguments =     (
        "/System/Library/CoreServices/backupd.bundle/Contents/Resources/backupd-helper",
        "-auto"
    );
    RunAtLoad = 0;
    SHAuthorizationRight = "system.preferences";
    [color=green]StartInterval = 14400;[/color]
}

Easy, huh?
...Or if you are lazy and don't want to do the interesting stuff above, just download time machine editor.