Usually pour it out the night before. But how long is this water "good" for? Let's say 7 days later, if I adjust the ph, can I still use it? OR Should the water be used quickly?
Yes, water doesnt go bad as long as you don't get "stuff" growing in it, like gnats, flies, mold, etc etc. It always helps to aerate the water before using it though. Just a tip.
I have an 18 gallon water res in my basement grow. its easier than making 10 trips up and down for watering. I have 2 large air stones running 24/7, and its in a light proof container.
DemonTrich is spot on. Keep the water moving via air stones/pump and use a dark container with a dark lid on it. The air pump will also help with keeping the water saturated with O2 which will be great for the root zone.
You sir are right on the money, although you are difficult to understand. H2O does not ever go bad. It is a vary stable molecule. Now, water that is not in a vacuum will contaminate. How long it takes to contaminate, you will need to ask someone else as I do not use hydro much.