I have a home depot bucked that I marked with .5 gallon graduations, so I usually fill it to the 2.5 gallon line and add .5oz star san (there is a .5oz line on the star san measuring portion of the bottle). Thats usually more than enough to get you through a brew day.
If you use tap water, it will last a day or so. If you use distilled water, it will last until the end of time or until you contaminate it enough to alter the pH.
One large bottle of star san, using .5oz per brew day/bottling day/whatever, lasts me like 8 months so I never worry about storing and reusing it, I just dump it out when Im done.
When it comes to use, at normal concentration a 30 second contact time will kill 99.99% of anything living on your gear. So you can immerse parts in it, pour a couple cups into a carboy and shake for 30 seconds, put it in a spray bottle and mist down hard to reach gear, etc. It's totally safe, no-rinse, and you dont need to worry about the foam it creates when shaking around in a carboy.