Maybe 2000 sf per set of pads. Depends on the carpet fiber stiffness, and soil load. After 1000 sf or so, tip machine up and check on pads. Don't let them get down close to the drivers. Plus, the pads can build up fiber boogers and become ineffective.
Or you'll be sending Ricky about $250-$300 for a new set of drivers......and you can damage carpet if they wear too low.
With Rickys package deal on pads, you can buy both grey and beige pads. I find the grey pads to be too aggresive on some CGD. With both kinds, you can judge for yourself.