I let mine sit and mix for 48 hrs. I'd do 24 hours minimum. Not only does the salt need to mix with the water to dissolve, but from what I understand there are also chemical reactions that take place with the salt and water. And you need time for that to settle down.
125 gal. reef w sump/refugium & 8-bulb 640w T5's
45 gal fresh w/plants + CPF lighting