a few months. there's no way to say how long it will take, though. it's up to nature, your clowns, and your nem. if the nem is too small and new, i'd be really hesitant to add it to a tank with clowns. you want to allow the nem to get healthy, adjusted, comfortable and of a good size prior to putting it in a situation where clowns are involved as clownfish can easily love a nem to death. a nem that is recently added is even less likely to endure this attention and much less one that is small.
but if there's no way around it, then perhaps you'll luck out and the clown won't pay any mind to the nem for a while allowing it time to settle and grow in size.
Same with mine...Dixie girl told me try placing anemone a little higher...sometimes clowns don't host low....I tried it and I'll wait and see..but both my clowns are small compared to my toadstool leather or torch coral