I recently listend to the 2 Brew Strong podcasts (Cleaning & Sanitizing) that had a representative from 5 Star speaking. He wouldn't commit to an actual length of time that it is OK to store Starsan in a keg, but said that he had done so for 6 months with no issues, & that being acidic, was probably safer for the metal than storing water. I personally do it every time with my kegs.
By the way, Starsan made with RO or distilled water will last nearly indefinably.