Some researchers have claimed that typical household food scraps are reduced up to 95% in volume. Manure is closer to 50% I believe. Using the 95% figure, every gallon of food scraps you add only results in a little over 3/4 cup of compost. At that rate it wouldn't be surprising to see a small family only needing to empty the compost from a 2 foot diameter unit every few years, but to be conservative I would say annual emptying is likely.
