It takes an egg about 25 hours to make its way through a hen's internal egg laying factory. The egg can only be fertilized during the first 15 minutes or so of that journey. If the mating takes place in a Monday, Monday's egg is not fertile.It started its journey on Sunday.Depending on what time the mating took place and the egg started its journey, Tuesday's egg might be fertile.But it might not.Wednesday's egg is almost certainly fertile. Notice that this is after a mating. A rooster does not always mate with each and every hen in the flock every day. Usuallyy a hen will stay fertile about 2 weeks after a mating, sometimes a little less but usually a little more.ut it might not. Wednesday's egg is almost certainly fertile. Notice that this is after a mating. A rooster does not always mate with each and every hen in the flock every day. Usuallyy a hen will stay fertile about 2 weeks after a mating, sometimes a little less but usually a little more.