How long should I mix my salt for?

Ideally it should mix for about 24 hours to be sure all the salts and other minerals have time to fully dissolve.

Raw undissolved salt can hurt all kinds of things.
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.