How do i know if my budgies are on an 18 day breeding cycle or a 21 day one?

 

My hen started sitting on eggs on Aug 25th, when are the eggs due to hatch?

When the hen lays her second egg, normally that's when she will start sitting, So I always mark it down from when the second egg is layed. :nest: That should be 18 days. sometimes they hen might sit earlier but not often, and sometimes the hen doesn't sit good and it can be more, But 18 days is the cycle. I hope this Wayno.