I have found that distilled water is not as good to use once it has been sitting around for a bit opened. Fresh is best. Quite frankly I was amazed at how the readings changed after the bottle had been open for a time. The only thing I can think of is that something leches out of the bottle material into the water. But I'm not sure. So now I get the best distilled water and use what I need right away when I open it and the rest goes for something other than making CS.
As for how long it takes, I have never had a batch take less than 8 hours so far (I'm pretty new to this) But each one has been Crystal C. and meters out to about 20. I use a Silver Puppy as well, on auto.