I have a 14 week old puppy and I go through about 2 pads in a 24 hour period. I change it when there are very few "clean" spots left on the pad. This also seems to be around the same time that the pad begins to smell. Everytime I change a pad, I make sure to press the new pad into a recent pee spot on the old pad so that the smell transfers over and she knows to go there. It has worked great so far... she has NO pee mistakes off the pad!! Now if only I could get her to poo on the pad 100% of the time...
