Tossing it is the safe answer, usually down the toilet or a drain in the basement/GR if you have one.
If all of your plants are fed and none will need water for more than 24 hours after that - I'd toss it. I've never been one to let my water sit for more than 24 hours and use it again, if it's only a cup or enough to water a plant or two, I'd dump it.. but if I have enough for 4+ plants after watering, I'll keep it up for up to 24 hours if I feel another plant may need it before then.
Check the pH on it again before you use it, if it fluctuates extremely from the original pH, toss it. Look into getting smaller containers if say you're using a gallon size container and you only use half, get a half gallon container.. etc. If you're using a gallon and using 3/4 of it, what I'd do is get a measuring cup and measure out where 4 cups of water is and leave a mark on the jug. 4 cups = 1 quart, so 12 cups would be 3/4 of a gallon and then you can mix your nutrients on that scale... hope that's a clear enough explanation!