Jell-O usually takes a minimum of two hours to set if placed in a refrigerator. This time is just an estimate and depends on several other factors, such as temperature and additional ingredients added.
The setting process for Jell-O depends greatly on the temperature at which the Jell-O is stored after mixing. Cooler temperatures will result in a faster setting Jell-O. Adding a couple of ice cubes instead of cold water can also help speed the Jell-O's setting. Adding any fruit or vegetables will cause the Jell-O to take longer to set. For best results, allow the Jell-O to set overnight, especially when adding fruit or vegetables.