If you are talking about the left over mixed up fertilizer that you did not full use after making it into a feeding solution, it is good until it starts to change, this could be colour, smell or forming precipitate. Things will grow in the water and change it like algae or fungus like yeasts and molds. Good rule of thumb use it within a weeks time.
If you are talking about the bulk concentrated bottles your dilute into bigger jugs of water, its good for years. Ideally 2-5 max unless its a dry powder being reconstituted.
If your talking about collected runoff from watering your plants and wondering about that, I'd day a few days. However it's not best to use that stuff unless its outdoors. Can contain a lot of excess salts.
Best Of Luck & Happy Growing!
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