It usually takes about 18 - 22 wks before a pullet is mature enough to lay, but that is just a guide line. All breeds mature at diff. rates, so it depends on the breeds, their feed, and also their environment. All my pullets are the same age and they are maturing at diff rates even among the same breeds.
The pullets usually lay small, cute, petite eggs at first that gradually get bigger within 2 - 4 weeks. Sometimes their first egg maybe a very thin membrane egg that is easily popped.
Since your chics are growing up through the fall and winter months I'm not sure how that will effect when they will begin to lay, maybe a more experienced person can asnswer that question.
I received my day old chicks on April 11 and they turned 24 wks old on Wed. One of my SLW was the first to lay at 20 wks old, but I have a few SLW and several Ameraucanas that have not begun to lay yet.
Hope that helps.