Depending on the method used to grow salt crystals, formation of crystals occurs within several hours or a couple of days. Both methods require boiling hot water to melt the salt.
For smaller crystals that form in a matter of hours, soak cardboard in the hot saltwater mixture. Once soaked, place the cardboard in a dry sunny location. Small crystals form in just a few hours.
A large crystal forms when the cooled saltwater solution gets a seed crystal added to it. This seed crystal dangles in the solution from a pencil or other level object. Place it in a cool and shaded area. The large crystal forms in a few days.