+1^
I've used Starsan that had been mixed several months before. It's all fine. I actually rack the Starsan carefully to another bucket when the previous one has a lot of dust and residue on the bottom (it looks grayish), leaving that last murky pint (or quart) behind. As long as the pH remains 3 or under it works as intended, so don't put alkaline-soaked things in there, without first rinsing off (e.g., PBW, Oxiclean, washing soda).
One thing, placing a lid on the bucket is good to keep dust and stuff out and prevents evaporation, but after a few weeks you may get some (black) mold growing on the inside of the lid and areas that are not submerged in Starsan. It comes from condensation forming on those surfaces. So, wet/wash those surfaces every week or 2 with the Starsan to prevent mold from forming.
Fill a spray bottle with your Starsan solution, good for spot sanitation and under rims of bucket lids, etc.
Now preparing 2.5 gallons instead of 5 may be more efficient, making a fresh solution more often.
