How do you define "colonizing"?
If you are thinking about sheer size, it depends a lot on the kind of coral and how well it's kept. Most corals will grow slow but then begin to accelerate in their growth after they have reached a certain mass and had enough time to really acclimate to the tank. Common Toadstool leathers will grow massive pretty rapidly, while other toadstools (ie Tyree toad) will grow much slower. Some acros grow painfully slowly, while others will settle in and grow like weeds (of course the weed like acros are usually the brown ones but that's another conversation....)
If you are thinking more along the lines of seeing the same thing popping up all over the tank, a lot of the time that comes from fragging either intentional or by accident. For example if you have a branching monti digi they often shoot branches out all over the place and after a while it's almost impossible to avoid breaking chunks off during maintence and whatnot. Pretty soon those lost chunks start settling and you begin to see the same coral sprouting up all over the place. On the other hand, if you actively frag your corals as they grow, you have to put the frags somewhere and then they gradually start to grow out.