What is the gestation length for a chicken?

Your question is based on a misconception. Chickens do not give birth to live young through a gestation period like mammals. Instead, chickens reproduce through a process called oviparity, where the female bird lays eggs that develop and hatch into young chicks. The gestation period for chickens, therefore, does not apply.

A hen normally takes around 21 days to hatch the eggs she is brooding, but this can vary slightly (it can take from 18-25 days).