In the spirit of your question, on reuse of cleaning and sanitizing solutions, a Starsan working solution can be used for weeks, even months after mixing it. Even when it gets cloudy it still works fine. I always have a bucket or 2 with Starsan sitting around and a spray bottle for spot treating areas, such as around airlocks, bucket rims, jars, lids, etc.
Aside from just dunking things into I also keep a few small washcloths in those buckets for mop-sanitizing items like side walls and rims of fermentation buckets, bucket lids (the groove!), around jars with yeast before opening, and so on.
I make a new Starsan solution when the old one gets grayish and too grimey to my standards. As long as the pH remains under 3.0 (pH/indicator strips) it's active. As long as you only use it to sanitize clean items, there's little reason for the pH to rise.
Cleaning solutions like Oxiclean and PBW lose their O2 relatively fast. But the remaining solution, which is now mostly washing soda (and metasilicate in case of PBW), is still a very good cleaner. IMO, the acclaimed O2 action is highly overrated through aggressive and annoying TV advertisements.
I keep some of those used, but still fairly clean cleaners, in a bucket or a couple plastic containers (with lids) for later use, such as cleaning fermentors, buckets, kettle, plate chiller, hoses, etc. Most cleaners work much better at higher temps, so use that to your advantage where it counts. Use "common" sense.
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