Set the dipped truffles in a dry, coolish area for them to dry and harden.
A temperature under 75°F should be cool enough, so long as the atmosphere is not overly damp.
How long it takes depends on temperature, type of chocolate, and ambient humidity. Drying time may be anywhere from 15 minutes to some hours.
If the ambient atmosphere has a high humidity level, it will take longer to dry. The outdoor humidity can affect the humidity in the house.
Do not dry the truffles in a refrigerator unless they will be consumed within minutes of being removed from the cold. The reason is the high humidity inside the refrigerator. The cool temperature may harden the coating quickly. But once removed and brought into a warm environment, the coating soon will get sticky.
Patience generally is best.
When dried too fast or in a high-humidity environment, the cocoa butter in the chocolate may soon start to slowly separate; which changes the dipped candy's color to (generally) a lighter shade of brown or grayish, usually in spots. It is still good to eat — it just looks less perfect.
Patience in drying and proper storage (cool, low-humidity) tends to result in candy that remains attractive for a long time.
