I set my own personal record for the "most soiled" carpet recently. I did a 2 room job and pulled a full 36 dirty pads. These were not just lightly dirty pads, even the 36th was dark, coffee brown! I would have kept going, but it was getting ridiculous. Unfortunately, no before/after pictures of this one, but let's just say the carpet went from dark dark brown to off-white. Luckily I had 50 pads with me, otherwise I would have run out... The day before I only used 25 pads on a 2000 sf. job.

I've had a few "pad-eaters" like that from time to time. Most often it seems to be in the homes of fairly elderly people who have never before had their carpets cleaned. Carpet soil is kind of like the half-life of uranium. There's always half left from what was there before. It's never totally gone. If/when you run into one of these situations again, ask the customer if her level of expectations is reasonable. They usually say "yes. Just do what you can." If the customer's expectations strike you as unreasonable, I'd leave this job to someone else.
You'll learn after a while "when to hold them, and when to fold them".

no matter the method OR machine, some soil is ALWAYS left in the carpet.

thanx --- D