Most people wait at least a day. When i do waterchanges I usually start the water mixing the night before, or the morning of. Then in the evening when the kids go to bed, i change the water. There have been a few times that i've done it a couple of hours after mixing it. I've seen no negative side effects either way. I use coralife salt because i was given a 5gal bucket full of it.
The issue with using ASW that is freshly made is the time it takes for the buffer system to develop as CO2 comes to equilibrium in the water. The stronger the agitatin of the water, the more quickly it can be used. Craig Bingman used a paint stirrer on a heavy-duty high speed drill to mix some ASW and found that it will come to equilibrium within 15 minutes (that is a bit of overkill, but it demonstrates the concept). I normally wait 24 hours for the buffer system to develop using a 55 gal brute on wheels and a mag 7 to mix the water with a heater in the water. Using this method, you usualy wil see 8.2 pH in the water and 80F within 6 to 8 hours, but being as I am a bit more cautious, 24 hours is quick enough for me. This becomes more important as the volume mixed becomes larger. You can prolly mix a 5 gal batch of ASW with heavy agitatin in an hour or two, test the pH after some agitation and see where the value is. If you don't see pH = 8.2 and Temp = 80F (or whatever your tank value is), then let it circuate for a while longer.