The biggest improvement in derailleur pulley technology in the past few years has been the introduction of ceramic bearings. With ceramic bearings, as offered by SRAM or Shimano, the claim is that 1-3 watts of power is saved. When coupled with the claims of wattage savings for ceramic bearings in bottom brackets, which ranges from 3 to 8 W, you end up with a drivetrain functioning at roughly 10 W more efficiency than one with standard steel ball bearings. Derailleur pulleys on your bicycle, be they should Shimano, SRAM, or Campagnolo, generally will last longer than even five bicycle chains. Still, when derailleur pulleys get all gunked up, or those with sealed bearings start to not spin as smoothly, it's time to replace them. Your rear derailleur, if it's a standard one, which 99% of them are, will require two derailleur pulleys. Furthermore, they are generally identical, that is, they are interchangeable. Some derailleur pulleys do have directional arrows on them to indicate the forward spinning direction.