Hi,
Under normal operation brake pads are pushed against the disc rotor when you press the brake pedal, the calliper responds to the pressure of the fluid and applies the pad to the disc.
When you release the pedal the pressure comes off and the calliper retracts.
If the calliper bushes are sticking or there is a buildup of dirt/grime/rust, then they don’t retract properly leaving the pad just sitting on the disc and slowly wearing whilst you’re driving.
Usually when you change brake pads it is customary to check clear operation of the calliper before fitting new pads.
If they don’t travel freely, then you would either clean them or fit a new calliper kit before fitting new pads.
Secondly, there are also varying degrees of quality in brake pads so some will wear faster but provide better braking response, some will last longer but won’t be as effective.
A good indicator of brake wear is the amount of brake dust sitting on the wheel rim, if you wash your wheels and they develop a good deal of dust on them quickly it’s a fair indication that either the callipers are not retracting fully or the pads are the quick wear type.
If you are doing brake pad changes you should also inspect the disc rotors for signs of scoring as this will affect braking and disc pad life.
If the disc rotors have signs of scoring or damage they should either be re-surfaced or replaced if below the minimum thickness level.
Hope this helps