This has been beaten around quite a bit over the years. Palmer was wrong if he said that it's effectiveness was tied to the solution becoming cloudy or not. I had a discussion with the president of 5 Star who told me that if the solution is kept free of organic material (meaning that you thoroughly cleaned the items before sanitizing them), that the only factor that would determine it's effectiveness was that the pH of the solution was below 3. The cloudiness can be caused immediately if used with hard water, but that it's still effective.
Most people, me included, keep a bucket of the solution for weeks, if not months. I've even "sweetened" it back up by adding some more concentrate to the solution I already had to drive the pH back down below 3. Been using Star San this way since I began brewing almost 7 years ago. No infected batches to date.