If you use distilled water and keep it sealed, it will last for months. When you use tap water it becomes hazy in a few weeks generally indicating (but not always) the pH has changed and it is no longer an effective sanitizer.
I have been working off the same 5 gallons for almost a year (10 brews or more) and I still have 2+ gallons. If your equipment is cleaned thoroughly before sanitizing you can pour it back in your storage container when you are finished.
Two things in closing
1. A $.99 spray bottle and wallpaper tray are your friends. The spray bottle is handy when taking hydrometer reading among other things. The wall paper tray is good for long spoons, hoses and two handed bottle sanitizing. Both can be found at your local big box hardware store for less that $10.00.
2. Don't fear the (starsan)foam! You can search this site for a number of threads on this topic.