BNOWs lay one to up to a dozen or more eggs/year/per clutch (sometimes they have more than one clutch/year). A ‘clutch’ is a group of eggs laid in a nest at one time. Larger clutches sometimes result in not all chicks surviving. Two common reasons for this: 1) Not all the eggs always hatch. 2) When the female lays the first egg she begin incubating it, meaning development begins. It takes her 1.5 to 2 days to lay the next egg. So, for example, if she lays 7 eggs, it may have taken her two weeks to lay them all. They will hatch accordingly, each one after approximately 30 days. This means by the time egg #7 hatches, egg #1 (if it hatched) will already have 2 weeks of development under its belt. BNOW chicks can fly at around 8 weeks, so 2 weeks is a lot of development time and younger chicks are sometimes not strong enough to compete with larger, older siblings for food.
It’s approximately 30 days.