If for some reason the temperature has dropped below the correct temperature on a few occasions, you could find some of your eggs only hatching on day 22, 23, or even on day 24 or 25. Whereas fertilized chicken eggs usually take exactly 21 days to hatch, most duck eggs take 28 days, apart from Muscovy ducks. As a general rule of thumb, the eggs of Muscovy ducks take 35 days, but once again, it can vary slightly depending on conditions.