Incubation period (days) in chicken (Gallus gallus) is

Incubation period (days) in chicken (Gallus gallus) is 21.
Fertile chicken eggs hatch at the end of the incubation period, about 21 days.
Development of the chick starts only when incubation begins, so all chicks hatch within a day or two of each other, despite perhaps being laid over two weeks or so. Before hatching, the hen can hear the chicks peeping inside the eggs and will gently cluck to stimulate them to break out of their shells.