I would expect a set of brake discs to last 60,000 - 80,000 miles, but this depends on a lot of assumed factors...

Driver style plays a big part in brake wear. Driving fast and braking hard will obviously shorten the life of a set of discs. Likewise, if you live in a hilly area, you may damage or warp the discs by sitting on the brakes too long and allowing them to overheat. Rust can also damage the discs. If a car is left standing outside for long periods, the discs may develop surface corrosion and cause surface pitting and this will eat away at the metal and the brake pads. Likewise, if a workshop uses an air wrench with excessive torque and does not span the junction, it may twist the disc.