There is no time period they may pair in a day or 2 or a month, but even if they decide who is going to be the dominant clown (female) they still may not ever spawn. Tank conditions can be perfect and no spawning may take place, area and conditions ( Habitat) all play vital roles. Try getting a book called saltwater fish for dummies, it's a great book and can give you excellent tips on breeding. Good Luck
about one month depending if you have the right conditions in your tank.
make sure the clownfish are both of the same species and that a host rock or nemone is provided to spawn on