As we all can see from the different answers and methods......I don't think there's one special rule that holds true with this question.
I, myself use tropic marin pro reef and only let it mix for about 2hrs max before testing and doing the water swap......and everything looks great. I think it really has to do with the brand of salt too. I was using reef crystals for years and if i left it ix overnight, it always had this brown residue on the buckets....it left a brown spot on the towels i used to wipe the buckets down afterwards. I was always curious what that was.
Now that i use the tropic marin pro reef....it's crystal clear after about 30 min. ...and it's never left any residue like the RC did. not to mention the corals are growing like weeds now.