How many days of incubation does it take for a chicken to hatch?

Usually, if temperature and humidity levels have been ideal, the hatchling will start to break through the shell 21 days after the eggs were first set.

However, this is a 'rule of thumb'. Sometimes it takes longer and sometimes shorter times - I've had a hatch as late as day 25
This chick hatched at day 25 and is now a perfectly healthy Light Sussex.