Bout 24 hours if your talking about the extra things that are put into Reef Crystals
If you store Reef Crystal in a pre-mixed container, for long periods, then some of its extra stuff added will be go bad before you use it. It will also leave a heavy crust on your storage holding tank.
Something like regular Instant Ocean, or / Red Sea Pro (for evaluated numbers) like Reef Crystals would be fine, without losing the extra stuff or leaving a crust.