According to my Service Advisor a good rule would be every 2 replacements of the front pads the rear should be replaced as well. Unless of course you have extra mullah then you can change both front and rear pads together all the time.

So 1st change just front brake pads. 2nd change of front , change front and rear pads. 3rd change of front skip rear again, 4th change of front change with rear pads. So on and so forth....