3 weeks (21 days) to 4 weeks is when you can be sure that the hen's eggs are fertilized by the current male that she is breeding with. But, I've always had the idea that if a male is repeatedly breeding a female and the male is fairly young but mature, his sperm will "flush out" the weaker sperm of the other male.
I have no proof to support my ideas but all hens that were exposed to another male have not produced chicks from the original male, even while being housed with the previous male.
ETA: If you are asking how long before eggs will be fertile by any given male, you can assume that within 2 days after breeding, her eggs will be fertile if the male is continuously covering her.