While it might not be ideal, I pretty routinely mix salt and use it almost immediately after it has cleared --10-25% WC. Again, if you can wait, you should, but it is not the end of the world if you use it quickly. I've certainly never damaged anything doing so. If you use a paint stirrer bit on the end of a cordless drill you can bring it to equilibrium equal to waiting over night in just a few minutes: http://web.archive.org/web/20011217234403/www.animalnetwork.com/fish2/aqfm/1997/sep/bio/default.asp
i usually would have a 30gal garbage pale full of r/o water with a magnaflow pump on the bottom mixing the salt and i would just let it circulate and take from it when i needed to for emergency purposes... i also would leave a heater in the pale so the water would even be the same temp as my tank, mix in some ph buffer and other additives and i had a 30 gal bucket ready full of water at all times ICE (in case of emergency)
I have from day 1 always mixed a 5 gallon bucket of water with salt til it is clear. I use to pour it right in from the top for a few years with no issues. Then I began just pouring it into the sump. Now I still basically do the same, except that I now use ro/di water, and instead of pouring it in I now use a power head to pump it into the sump. Still no issues.