Sistan, Kodak Photo-Flo and other final rinse agents have a short life once diluted due to mold and bacteria. As a stock the detergent is too concentrated for stuff to grow in. Once dilute it is like leaving a glass of juice out, it will have stuff growing in it in a very short time. But it really is not an issue as these agents are cheap only a little is used per session in the darkroom. A single bottle can last for dozens of batches of film.