Hi Erika, 

It takes 24 hours for an egg to develop from the moment it is released from the ovary until it is laid. 

This timeline can be extended for a variety of reasons, but 24 hours is the minimum. A hen is hatched with all of the ova she will have in her lifetime, these ova develop into yolks and each hen has a number of yolks at various stages of development present at any given time until they are ready to be released from the ovary and begin developing into a shelled egg. 

You can learn more here: https://extension.psu.edu/hen-reproduction