Most fertilizer components are inorganic mineral salts, with the exception in many being Fe-EDTA, but that's a very stable organic complex.
Assuming the pH is proper (i.e., not thrown off by the water supply), you keep it well sealed between uses to prevent evaporation of the water, and you shake it well before each use, it should be good for a very long time.
is it okay to keep it uncovered fora week? because i mix a 5 gallon bucket of fertilizer and water everyday from it. it gets used up in about a week. is that ok?