How Long Does It Take For...

[The Wolf Queen]

a hen's eggs to become fertile after being introduced to a rooster???

 

[skeeter9]

They become fertile within a couple of days. You can check for fertility at that point to be sure, but that's usually how long it takes.

 

[Ridgerunner]

Skeeter9 is correct. It is usually 2 days after the hen has been mated to a rooster. When you introduce a rooster and hen, they may not mate right away, especially if there are several hens in the flock or if the hen is more mature than the rooster.

If your eyes are pretty good, you can tell if an egg is fertile. This link shows how. Unfortunately, you have to crack it so you can't hatch it, but if one is fertile the ones that follow should be. After mating, the eggs normally remain fertile for two weeks.

Fertile Egg Pictures
https://www.backyardchickens.com/forum/viewtopic.php?id=16008