This linguistic confusion is why Americans call the Pennsylvania Dutch Dutch even though they're Germans. To review: this country is the Netherlands, its people are Dutch, they speak Dutch. There is no country called Holland, but there are provinces of North and South Holland.
In clean, uncrowded conditions in tanks or ponds, goldfish grow fast. They can grow to a few inches (1 to 4 inches) within the first year. It really depends on their genetics and living situation. It may take a year or two before they change color from brown to gold or whatever color their parents were.
Goldfish should obtain an adult size of 1.5 feet and need a POND. They should also live more than 20 years. Keeping them in tiny tanks (like 12 Goldfish in a 12 gallon tank) is killing them. If you want any Goldfish for a 12 gallon tank, go get a SINGLE Fancy Goldfish and give away the Comets. Email me if you have any questions.