As long as the the pH is in the low 2s it's still effective. Killing bacteria happens (this number will probably vary based on whos telling you and for what purpose) under a pH of 2.5. If you mix up a batch with RO or distilled water there wil be very little alkalinity to "use up" the acid.
I'm going from memory here as I did the math a while back... Properly mixed StarSan will be around 2.1. Let's just call it low 2s. As long as its still 50% w/w phosphoric acid, that was on the bottle last time I checked.