How long it takes any coral to attach varies a lot, depending on type of coral, method of attachment, water quality, how happy the coral is.....
As for Xenia, there's a few methods that work well. Some pinch it between a couple pieces of rock or rubble rock, other use "bridal veil" to drape over it and tie it down. The method I use, which works great for me, with all soft corals...is to use a wooden or plastic toothpick, push it right through the flesh and then poke it into a hole in your live rock or a piece of rubble. Then break off the toothpick close to the coral, leaving the small portion that goes through the coral and into the rock. After a couple of days, the soft coral will start to attach. Once it's totally attached, you can pull the toothpick out. I've even had Xenia and Kenya Tree grow right around the wooden toothpick.