When I wondered about weather or not my starsan mixture was good or not, I called the number on the bottle and asked how long it would it would be good. The reply I got was it depends on the water you use. He suggested that I use distilled water, and that I could test the PH to know for sure. Starsan is acid based, and as long as the PH strips come out bright red, that shows that the acid solution is still strong enough. The one gallon jugs of distilled water are not much more costly than regular drinking water. I have tested solutions that were a couple months old and they were still good.
Primary; Smithwicks Clone, Honey Brown Ale, Cream Stout
Secondary; I usually leave in primary 3 wks then condition 3wks in keg
Hophead double IPA
Nut Brown Ale
Kegged; Biermunchers Cenntenial Blonde