I can't give you days or inches only my own experience. When I got my first star polyp frag it was only about 2 or 3 inches in length. It took quite some time for it to cover the (about) 9 inch rock that it was on under 380 VHO lighting, now it grows much faster than when it was smaller. I have had to peal it off of my leather coral once last month and it is about 1/3 of the way to getting back up the stalk of the leather. Got a nice frag of it growing on a small rock now, time to trade to the LFS.
Thomas