At a minimum, you need about 8 hours to steep cold brew coffee. At maximum, you need about 48 hours only when steeping in the fridge.  Anything shorter than 8 hours will produce a watery, bland cold brew that has not had enough chance to soak up the coffee grounds' flavors unless you grind your coffee beans finer and add some agitation. Anything longer than 24 hours on the counter or 48 hours in the fridge will result in a bitter, over-steeped cold brew that will not be pleasant to drink.  The sweet spot for brewing cold coffee is 12-14 hours.