The GH nutes I use are already liquid. You don't need to wait for them to dissolve. FF nutes contain things that do not dissolve quickly or at all. Letting that sit would be counter productive. I would dump it in as soon as I stirred it. Hopefully, the air pump will keep the added particles suspended.
If you add Calcium Nitrate and Magnesium Sulfate, add the MgSO4 first which dissolves almost immediately. Then mix the Calcium Nitrate in a jar of water before pouring it in. No reason in either case to wait for anything to dissolve.
Some Calcium Nitrate has impurities that don't dissolve quickly and ends up floating on top of the water. I have used that in a DWC with no issues, but it did make the water look ugly.