When making things with chocolate the time it takes for the chocolate to set and become firm will vary depending on the thickness of the chocolate.
Most chocolate sets in the fridge within 15 minutes but sometimes if the chocolate is a bit thicker it can take as long as 30 minutes to set.
Put the chocolate into the fridge and then check it within 15 minutes and it should be set and firm but if not leave it in the fridge for another 10 to 15 minutes and by then the chocolate should be fully set.