There are a number of variables that can affect brake pad life...but primarily, the condition of the brake hydraulic system (particularly the front calipers), how much highway vs city driving you do, how heavily you use the brake pedal (i.e. do you use engine braking and/or stopping gently vs panic-stopping), and how much you tow (particularly in city driving).
I replaced my front pads about two years ago...these pads were on the truck when I inherited the truck, and I put 64K miles on the pads. Three of the four pads still had almost half of the material remaining

I only changed them out because the left side caliper had seized up on the sliders and wasn't retracting properly

and one pad went down to metal-on-metal.
That 64K I put on the pads was mostly highway miles, and I make extensive use of engine braking when I slow down, so YMMV.
Before replacing the calipers, be sure to lube up all of the sliding surfaces as much as possible!! That was my mistake...I put a light coat, and the coastal salt air ate through that and that's why my caliper seized. The caliper piston was fine; I just retracted the piston and put new pads (and rotors) on and made sure to over-lube the sliding surfaces, and all's been well since
