[mirkules]

You can change the frequency, but you have to use an external app. There are a number of ones out there, for example: http://timesoftware.free.fr/timemachineeditor/ or http://www.klieme.com/TimeMachineScheduler.html

Note: I can't remember which one I tried so I can't recommend one, these are just quick ones I found off Google. I do remember as I used them that despite scheduling it in the App, time machine would incorrectly display "Next Backup: Tomorrow" or something like that.