How long for rooster to fertilize out old rooster?

[Viking84]

If I have a flock of 15 hens and 2 roosters of different breeds, I would have no idea which Roo has fertilized which eggs. If I want to separate a couple hens and coop them in a separate run with one rooster. How long should I give those hens to lay eggs that are certain to be fertilized by the rooster they are cooped up with?

 

[DobieLover]

If I have a flock of 15 hens and 2 roosters of different breeds, I would have no idea which Roo has fertilized which eggs. If I want to separate a couple hens and coop them in a separate run with one rooster. How long should I give those hens to lay eggs that are certain to be fertilized by the rooster they are cooped up with?

To be sure, at least 3 weeks.