My experience is on an Astra, obviously. OE pads on the front were that hard they broke up after 20k, replaced with Apec which lasted another 15k and still had plenty of meat left on them when I sold the car. I'd expect to get about 40-50k from them.

Rears, the Astra is the same as the Vectra in that it eats rear pads. OE pads lasted 20k until they were nearly to the metal. Replaced with Apec pads and discs, another 20k and again the pads were about 20% worn, the discs were hardly worn at all, and overall braking was much improved over the OE pads.