The simple answer is...when they get ready to. I've had a 2 week old crower (turken) and a roo that didn't start crowing until he was 15 months old (brahma). I currently have a polish cockerel that only crows once a day and he is over 4 months old.
We currently have a 22 week old Blue Wheaten Ameraucana roo who has not crowed yet! I think that he heard me say that he can stay until he crows! He is just flying low under the radar!!! I guess that he is smarter than I thought!
My neighbors Barred Rock started to crow at 18 weeks. I love it and I hope they keep him.
We have neighbors threatening us about our chickens. Those neighbors have zero tolerance for crowing no matter how faint or far away it is. I can only hear their rooster if I'm outside, he is very quiet.
I bought a few chickens and a rooster. They are all pullets, or let's say 16 weeks old. I know, the rooster ain't one. I got them home around 10:30 this morning and about 12:00 with them in the run the rooster started crowing. He is a Golden Laced Wayndotte. (sp) To my suprise his crow was a deep, throaty adult sound. I certainly did expect him to crow and I don't now why he did, but I am glad he is. These ain't show chickens but I'm happy with um.