Baby peeps will hatch from their eggs about 21 days after they begin the incubation period. In an incubator, this number can be easily monitored. However, when a laying hen who hatches her own eggs, she will first lay a clutch over a period of a couple weeks and then begin sitting on them all at the same time.ince it takes about 7-10 days for the sperm to travel to the ovary where they fertilize the ovum as it is released, a chicken won't have fertile eggs for at least a week after she is first mated. She will continue to lay fertile eggs for about 6 to ten days after she has last been mated.