Do you know how long the SN remains effective after being mixed with water? If it were mixed in a closed water bottle, for example, how long would it remain poisonous?
That's a good question, idk how long but I would say long enough. It's also a question of the temperature. I would say if you have SN do some tests. Prepare the aquarium test and dissolve SN, the keep the solution at room temperature another one in the fridge another one sealed in a bottle" and test them everyday. This way it would be possible to estimate how long SN lasts after being dissolved in water.
In any case, it's best to keep it stored dry and cool and prepare the solutions only shortly before plan to use them That is also the safest and easiest way, imo.
how long would it remain poisonous?
It's poisonous as long as there are enough NO2- ions in the solution.