I keep Star San in a plastic spray bottle. I don't really care if the inside of the spray bottle is slimy. How would I know anyway? I've got a gallon in a glass bottle, and another gallon in a thick HDPE bottle. I probably won't keep it more than a month, about two brew sessions, just to be safe since I have no idea how long it will be effective.
What I'd like to have is a test to determine if it is still active and doing its job. Then I could keep it until it failed the test.