That would depend upon the species and largely upon how much you feed it. Most species grow pretty fast when conditions are just right for them and they're receiving tons of food. I know someone who has had colonies of T. micrantha spreading around his tank within months, but he fed his tank far more than most people would be willing to do. He never even had to feed his Tubastrea directly, they were fed mainly from swarms of copepods, amphipods, and mysis shrimp that he had growing in his tank.