Here is a thread done a while back with a reply from API about their test kits. From reading their response, I take it that they are good for the full time regarless of whether they are opened or not. Of course, they also say Stress Zyme aids in cycling which I dont believe so I guess you have to take everything a manufacturer says with a grain of salt.
I also checked Nitrites and they are 0 which kind of puzzles me, because I thought they would be presents in such high amounts of Ammonia. 