According to Old Orchard Aquarium, a goldfish bowl should be cleaned at least twice a week if the tank does not have a filter. A tank with a filter should be cleaned at least once a week.
Cleaning a goldfish tank is essential because the fish excrete large amounts of ammonia, which eventually poisons them if it's not removed from the water.
Gold fish can live in a bowl without a filter, but the fish can benefit from the increased moving air inside the tank. When setting up a tank for goldfish, it’s best to keep only one fish per every gallon of water. The larger the bowl or tank that is used, the longer an owner can go between cleaning the tank. It's best to do a partial water change than completely renew the water each time.