I have seen instances where it takes 6 weeks or better before the clown finally decides to take up with the host anemone. Other times it happens within minutes. Certain clowns seem more likely to form this relationship than others. Wild caught clowns are much more dependant on a host anemone than tank raised clowns also. Skunk Clowns seem to be the most dependant on an Anemone and do not fare well if not provided one. I have added clowns to tanks already housing clowns and have found that the best chance for success is to get clowns that differ fairly significant in size. If you add a clown close to the same size as what is in there you usually have squabble over dominance. If you add a larger or smaller specimen than what is in there, after a few initial days of bickering, it is decided that the big guy is the dominant one in the tank and they coincide peacefully from then on.