In my car's early years, I was averaging changing the front pads about 1.5 times for each time I changed the rear pads. In its later years, it was more like double (twice as often for the front as the rear). I think the reason the ratio changed was, in the early years I would run the fronts all the way down to the squeaker tab (around 2 mm pad thickness), and would sometimes replace the rears when they weren't that worn; in the later years, I would change the fronts before they reached that point (typically at 4 to 5 mm), and would run the rears down to around the squeaker tab (2 mm).