I am by no means an expert, but usually between 18-24 weeks. Just prior to beginning to lay, their combs and wattles will get really red, and when you reach down to as though to pick them up, you may see them squat in anticipation of a rooster. You may also see them start having interest in the nesting box, sometimes even trying it out. You may find their first eggs in a variety of places, however, before they get the hang of. I found one once on top of a bag of mulch that was sitting upright and leaning against a fence! They usually figure it out pretty quickly, however.
The decreasing daylight going into the fall can delay it, though. However, from some things I have read, the older they are when they start laying. the more the pullet weighs, and the bigger their eggs will eventually be....there was some study I read about it basically saying not to worry if they don't lay early, because the upside is larger eggs when they do.
I had a black copper marans late spring pullet who didn't start laying until the following February, and her first egg was 59 grams....a large egg. She now consistently lays extra large to jumbo eggs! I had a Barred Rock who laid early, and to this day she lays a small egg. Perhaps it is a coincidence, perhaps not.
It is so hard not to get impatient with them, though.
Thanks, BC ! A few of them are definitely doing the squatting thing when I approach them or hover over them so that seems to be a good sign. I placed a couple of golf balls in each nest and I have noticed that they have been moved around so someone must be nosey about the nests ! All appear to be signs of pending laying I guess. Thanks for the advice !