I use a 7 quart sprayer and mix it at the start of the day add more if I am running low as needed.....anything left over at the end of my day I dump out.....
If you haven't any oxidisers or enzymes in it, it ought to last a long time. Potassium and Sodium Hydroxide (high PH salts) used in these types of cleaners has in practical terms a very long shelf life, even dissolved in solution. D-Limonene is a similar case.
Unless you're O2 boosting, don't waste your left overs.
Flex I mix on Friday/Saturday, I use on Monday. Pump up sprayer. Still cleans just as well. I'm sure hot mixed Flex is more potent, but it works for me cold.