It depends on the temperature of the water you're using, mostly. If you boil the cashews on the stove, 15 minutes will work. If you soak them in very hot water on the counter, 20 minutes (or up to 2 hours) will be long enough to soften them. In room temperature water on the counter, soak them up to 4 hours. In room temperature or cool water in the refrigerator, soak them overnight.