My own struggle with soft corals has found highly variable results for attachment times.
The old addage, a watched anthelia never attaches may be true? I think a fast time (without glue) would be a week, and average time of around 3 weeks is when I have found soft corals to have firmly attached so that they don't drift away when I removed the rubberband, or other restraint that I might be using.
That's one guess, lets see what others propose. I know once it gets attached and growing it can move a long way in the same three weeks!