I mix and use it as soon as the water is transparent again. So what I do is, put the R.O water in to a bucket add the salt and in goes the powerhead. I then start to remove the water from my display and once that's done I add the new mixed water...
Not sure if you're asking about IO specifically or using any freshly mixed salt faster than 24 hours. Something I learned recently is that mixed IO can sit for a long time (weeks), while other salt mixes expire in days (organics). Some salt brands are ready to be used after 15 minutes (AF) and waiting 24 hours is not recommended.