How often are heavy aircraft brake pads changed?
The 747-800 uses carbon brakes made by the Goodrich company and according to this article,


  The entire industry is leaning towards carbon brakes, Honeywell
  executives assert. Lufthansa Technik also sees a trend towards carbon
  brakes, as the mean time between repairs (MTBR) for carbon brakes is
  1,500-2,000 cycles, compared with some 1,100 cycles for steel brakes.


Also of note, take a look at this video on the testing of the brakes.