At about four months my RIR hens started laying. I worried and worried until I got the first egg. It is so exciting. I still have the notebook where I listed how many eggs I got every day for the first couple of months.
Some people say 19 weeks but I think it is more like 22 weeks on average. It will happen when it happens and it will be so exciting!!
I've had sex links that started laying around 18-20 weeks, easter eggers that were 24-20 weeks, a barred rock that was 36 weeks, a wyandotte that I had for 10 months and never gave us eggs (we gave her away) and my silkie is around 40 weeks...still not laying. LOL
The youngest I have ever had was a little New Hampshire.. she started laying at 12 weeks right on the dot.. her daughters started laying at 12.. and their daughters started at 12..
all the others have been 16 to 24 weeks depending on breed