Re. how many sq. ft. of carpet you can clean with a single pad: it does vary a lot, but maybe 100 - 200 s.f. per pad might be a close estimate. It also depends upon the pad. The Challenger cheap-o pads are all-cotton and on the thin side - a sure formula for rapid wear-out. However, at the price they currently sell for ($3.00) value-wise, they are a very good buy.

I prefer 2-sided pads, like Challenger's 100% cotton ones.
These are virtually the same as were sold by the makers of the Argosheen c.c. product for several decades. I also have a lot of 19", single-ply, mostly cotton pads, which I have taken to a person who does commercial stitching and tailoring. I had her sew these pads into double pads. That way instead of having to replace a pad as soon as it has become soiled, all I have to do is flip it over to the clean side. Downside: it's a real handful (literally!) to try and squeeze out by hand, so I usually immerse them in one of those side-handle bucket-wringers to squeeze out the excess water.
With these pads, I can sometimes go several hundred sq. ft., if the soil is light-to-medium.

I'm a big fan of Enduro pads from Challenger I ordred some last summer and have yet wear them out completely. I use them primarily on CGD So they do take a beating For residential I have been using microfiber pads. The ones I use are big and hold alot of moisture and soil.They are not as agrresive as Cotton but they do pick up nicely. I have also used Glads but I got some out of the bad batch early last spring. The ones that stayed together worked great On my big rotary I use Tuways and micro's