The colder your ice cream base is when it goes into the ice cream maker the faster it will churn and the smoother it will be. This chilling period in the fridge also gives the egg proteins time to mature and thicken. The longer you let it sit, the smoother, creamier, and more flavorful your ice cream will be. Three hours is the minimum amount of time I recommend chilling it for — but if you can do it overnight that's really ideal.