I usually make 1 gallon in a separate bucket and use it to soak all of my hardware that needs sanitized. I also make up a spray bottle too for some of the smaller things. When I'm ready to transfer from the kettle to my carboy I dump some sanitizer in it with a funnel and swirl it around in there. There might be better ways of doing it, but I've never had an issue doing it this way.
I basically do it the way wildactbrewer does it. I make a five gallon solution in my bottling bucket that I soak all of my equipment in while my beer is chilling. I empty about a gallon of that into a carboy and shake every few minutes
Four ounces of sanitizer, like Starsan solution, would be enough to slosh around in a carboy to sanitize it. Then dump it out in a bucket to save it. As long as the pH of the Starsan solution remains below 3.2 it is good to reuse.
It's easier to make it up by the gallon though. I keep four gallons at the ready in emptied distilled water jugs.
I make a gallon at a time & store it in AZ ice tea jugs. Thickest walled plastic jugs out there for reusing imo. I pour a gallon of Starsan in the fermenter & swish it around a few times before filling. Then use a radiator funnel to pour it back into the jug.
You can reuse Starsan solution until the pH rises above 3.2, or you accidentally dump a bunch of crap into it. It will be cloudy, but the pH of the solution is the sanitizing part, not the clarity.
I use simple test strips from the home brew wine supply to periodically check the pH.
Yeah, I do it all the time. Since the fermenter is clean at that point, all you're doing is sanitizing it. Nothing in there to dirty it up. I'm still using the same 8oz Starsan bottle I bought when I started. A little goes a long way a gallon at a time. Fill the little measure on one side of the Starsan bottle 3/4's of the way up to the 1/4oz mark. Pour into a gallon of water & allow to settle through it. Done.
As was said above, you can reuse it until the pH gets too high. Without a pH meter, it's hard to know, and some will say if it becomes cloudy it is no longer good, but others who have poor tap water and get cloudy sanitizer right away have measured their pH and found it to be fine. Generally if you mix your StarSan into R0 water and keep it covered, it will last months.