Depends upon a lot. Different kinds of xenia have different growth rates. Environment of your tank influences growth too. Here is my experience:
I had xenia elongatta. It pulsed like crazy. If was one of the first few corals I had in my tank. My tank had a real nutrient overload problem because of the previous owner's neglect.
So, this xenia was happy as all get out. It doubles in size every month. This kept on for 7 months with the stuff growing over rock, and up glass. I pruned it and sold it to my LFS for whole price (about 1/3 retail). Made close too 400$ of the stuff plus traded lots to friends in the reef club.
Today my tank keeping skills are much better, and I have lots of competing corals in the tank too. My xenia is all but gone and won't grow anymore. Too bad, I liked it a lot. This is apparently a not uncommon occurrance.
Good luck, Kevin