what kind of corals? it really depends. encrusting gargonia a couple of days. along with some softies. SPS it is variable also with the species. mine usually start encrusting within 2-3 weeks. but some much longer.
In general, the slimier it is, the less it will cooperate. If you can get it to stay in one place, it may attach in a week. The problem is that they come loose. Colts are IMHO one of the most difficult corals to mount.
Good luck - be patient with it, and hopefully it'll come around,