Never. Once a year. 3 times a year. 24/7.
Depends on the chicken. I have a young Maran who seems to be thinking about it, but not seriously. She takes her time laying and gets all huffy if you want the egg, as tame as she is. The rest are in and out all business like. She started out that way too... but now... she may be getting ideas. Jokes on her if she tries, she's getting ducks.