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If its an emergency you could wwait like an hour. Usually people say like a day but I think a couple hours is all if you trully get it mixed. I use a pump in my trash bin so it gets mixed amazingly.
You want to make sure it is completely mixed depending the the pump you use will make a difference how fast the salt mixture dissolves. I have found some salts don't dissolve as completely as others even overnight. I let mine mix overnight at least.