If the solution only contains NaCl and KCl then there is nothing to support microbial growth. The solution should last indefinitely. A saturated solution is going to vary in the concentration of NaCl and KCl depending on temperature. A non-saturated solution would probably be a better choice. Since they have different solubilities, one of them (NaCl or KCl) will have a higher concentration in the water than the other, depending on the temperature. A non-saturated solution would be a better choice if you wanted to maintain a specific ratio of NaCl to KCl or know how much you are adding.