The time period starts after the rooster and hen have successfully mated. Just because they are together does not mean a rooster mates every hen in the flock the first day they meet. Certain breeds have such thick feathers that the rooster has difficulty completing his task, Cochin and Orpington are the two I hear mentioned most often in this regard. You can do a search on "vent trimming" for further info. Diet and age can effect a rooster's fertility. Some roosters have poor fertility due to genetics. Some hens and especially young pullets resist the rooster's advances, even if they are laying eggs. And some roosters are so clumsy, especially young ones, that they have difficulty hitting the target.
Just because they are together or because you see them in the act is not confirmation the hen will lay fertile eggs, no matter how energetic the rooster appears.