I've used WFP almost all the time since 2005.
When I cleaned traditionally, I just part filled the oblong bucket and laid a clean scrim on it to stop it slopping over in the passenger footwell. I never squirted detergent (I used Fairy) into the bucket. I just squirted a bit onto the applicator and dipped it into the bucket part way (you don't usually need it too wet), then frothed the sleeve up a bit with my hand. So long as I didn't put the applicator into the bucket, that bit of detergent (so long as it was a thickish one like Fairy) would last for a few houses. I did try other window cleaning liquids but always came back to Fairy. It's all very well these other liquids promising no suds, but I liked to see if I had messed a bit up so that I could re-do it. Also, I found that some other liquids didn't give enough "slip" on the glass.
If cleaning the above way, I could go the whole day without changing water too - and it would still be pretty clean as the only contact between dirt and water might be a light dab of the applicator into the bucket.
Obviously if doing leads or Georgian, when cloth makes contact with glass, it could be more beneficial to change water but, so long as cloths aren't wrung out into the bucket, it can be kept to a minimum.