In my experience four weeks segregation is best to be sure. With my hens I didn't see hardly any drop in fertility until the end of the third week, but all of them were infertile by the end of the fourth.
A breeder friend told me he found that after 1 week with a new cockbird, the hen is more likely to lay eggs fertilised with the fresh sperm received from the new cockbird. I haven't tested this, but he did and said it worked well for him.
I wait at least a month before collecting eggs when I've introduced a new rooster. I've also heard that a hen is more likely to lay eggs fertilized by the new cock after about a week, but the girls can hang onto to sperm for a bit, so I always wait a month or more to be sure.
Well I just might try waiting a week or so and see what happens. Interesting that they will use the new sperm that soon. Looks like I'll be doing some more research on that its got my curiousity going now.