When do I need to cut out ventilation holes?

[chickierookie]

I have a homemade incubator and i want to know when to cut ventlation holes

 

[Goblnfvr]

Ventilation holes are needed from the start.
They allow a steady flow of fresh air into the incubator during the incubation period.
I have 9 holes that are 1/4" in diameter in my home made coolerbator. 7 around the bottom and 2 in the lid