I keep StarSan in a spray bottle for easier sanitizing during brew day.
How long does the StarSan last in that bottle? Is it good forever?
Just after posting the question I found the Tech Sheet on Five Star's website. It says that the sanitizer is good so long as the pH remains below 3. Since I'm keeping it in a spray bottle with distilled water it shouldn't ever get above 3.
StarSan along with many of the other no rinse / food safe sanitizers are primarily just acid solutions. Of which the largest proportion is Phosphoric Acid, with some other stabilizers and minor acids depending on the make. These won’t naturally decay in any medium time frame and as long as it stays acidic I.e its pH is maintained in the acidic range < 3.
What will affect it is:
The factor at which you dilute it, if you add too much water it looses its acidity and will not be effective.
What you dilute it with, if the water in your area is highly alkaline (brak) then you have the potential to, even at normal dilution rates to cause the solution to no longer be acidic, meaning it’s ineffective.
Case in point. It will stay as potent as the day you made it given (1) and (2) and no other outside dilution takes place.