Normally you don't really want to mix it up until your ready to use it. But if you keep it in a cool (75 degrees or so and below), and dark (no light at all) place, adjusted the pH, and use a air
pump and air stone in it to keep the water moving and mixing, it should be fine. But I don't know why you would be needing to mix it every few days. I often mix up one gallon of
nutrient solution in a water bottle I keep to use it to water my seedlings. I don't use it full strength. I mix the full strength solution (I have in the water bottle) with fresh water in another container to dilute it before I use it for seedlings.
When replacing the water the plants drink up (every few days) you want to use fresh plain water withough't any
nutrients. Only mix up a new batch of nutrient solution when you do a nutrient change. Sometimes if you plan to put off changing the nutrients longer than you should, I might add a diluted amount of nutrient solution. How diluted depends on how long I plan to wait until I do a nutrient change as well as how long it has been scene the last one. As well as if the pH is fluctuating, how big the plants are, and how much water volume I have in the reservoir to begin with.