A chicken sitting on a fertilized egg can expect to see a baby chick in 21 days. There are certain circumstances that can alter that time from 19 days all the way to 25 days.
Eggs that are hatched by a mother hen are kept at the perfect temperature, so hatching at 21 days is almost an exact science. Eggs that are hatched in an incubator often hatch a few days earlier or a few days later than the 21 day standard. If the incubator has a temperature set too high, the eggs will hatch early, sometimes as early as day 19. If the temperature in the incubator is set too low, the eggs can hatch a few days later than the standard 21 days, sometimes making it as late as 25 days.
Chickens lay eggs with or without a rooster, but the only way to get fertilized eggs is to have a rooster in the coop. In order to tell if an egg is fertilized it either has to be broken open or candle tested. Breaking the egg open there is a small white or grey dot in the center of the yolk with a dark circle around it that resembles a bulls-eye. If the egg is left alone for several days, tiny signs of development, such as veining, can be seen by placing the egg on top of an empty cardboard tissue roll and shining a powerful light underneath in a dark room. By day seven, there should be signs of a baby chick developing.