How Long Can You Keep Mixed Nutriant Water Out For??

Gday fella's .

I got a guerrilla type grow going on , and its a pain in the ass mixing up little amounts at a time and going to the spot to water .

How long could you leave mixed nutrient water out for (out in the sun to )

Other option is to take a big water container down and leave it near my plants , and mix when i get to them . how long would the water be good for out in the sun ?

water would go stagnant really quick because you cant use a air stone unless you run one of a car battery sure it would last long then with well maintained airstones ??