techincally, it should be 24 hours... But in my reality on my 120 gallon tank,... I just mix it for 3 minutes,I use my hand and high presure water :D :D
I mix it for 24 hours for two reasons. 1 make sure everything is dissolved. 2 while it is mixing I have a heater in the new water, this gives the new water a chance to come up to temp.
I use about a 250gph power head. An air stone. And a heater to get the water to 78 degrees. Like my tanks. And let it mix for 24 hours. Sometimes a couple more if I get home later lol
I usually go for 24 hours for the same reasons as Ted. But in a pinch, I've used saltwater after a couple of hours being stirred by a strong powerhead and a heater. But I've never done it without making sure the temp matches my tank.