How often do you allow your child to eat cake?

My sister lets her child eat cakes and sweets on special occasions, of course, but also occasionally during her snack time --- but only during snack time. This way, she doesn't go too crazy when a cake finally comes along and she'll eat so much of it. What's more important than "when" is "how much" so my niece knows that she can eat it sometimes but she can't have too much of it or else she'll get sick.