Under ideal conditions chick's will hatch in 21 days. So it isn't really like humans where there is tons of variations. But one day with too much time of the nest sets the eggs back and there are many variables and each hiccup adds to the time line. A few really wet days is a setback.. so on and so forth. Anyway sometimes eggs take longer, or shorter, it's all alright. I had 2 hens go broody last year and vowed to never again allow a hen to set eggs. I had 2 hens each sitting 8-10 eggs. At the end of around 28 days the chickens were still sitting and I decided to intervene. Alk the eggs were bad. Not one out of the bunch hatched. It was too hot. I suppose. So I gave them each another 8 eggs. On day 20 I heard peeping and got so excited. The next day there were no chick's but a bunch of shells. Later that day there were 2 chick's. Later- later that same day there was 1 chick. The 2 hens Co brooded that 1 chick... but when I set 16 eggs in my incubator I usually end up with close to 16 chicks. I'm better at it than my hens. I sold one of the hens and the sole survivor this past winter, but kept the other broody sister... and last week she started hiding eggs somewhere in the woods--now I see her every once in awhile she comes to get food and water and then back to her secret woods nest... hopefully she will fare better at brooding chicks in the woods than she and her sister did in the coop. Abs hopefully nothing eats her in the next few weeks while she's sleeping in the woods.