It takes approximately three weeks for chicken eggs to hatch, give or take a day, providing the eggs are fertile. Not all eggs are fertile, so time should be spent determining which eggs are likely to hatch and which eggs are not.
Infertile eggs contain small grey and white dots on the yolk, whereas fertile eggs have a darkish circle surrounding the dot. Fertile eggs need constant heat, naturally or from an incubator. Hens sit on the eggs until they hatch, protecting the bitties inside, while they are developing. Temperature and humidity affect the outcome of eggs and can destroy the eggs.