You can't ever be too certain that they will or won't, and there isn't a set amount of days/weeks/months until they do. Just because you have an anemone and a clownfish won't guarantee that the hosting will happen. MANY clowns host many things other than anemones, i.e. Coral colonies and even Clams!
My B/W Ocellaris started hosting a small Seabae anemone after two months of it being in the tank. Yet, once the hosting began, the anemone grew from a .50cent piece size to his now 1.5' or more across!