Ive heard somewhere around 9-11 weeks. But they all mature differently. Mine just growls and chases right now. He;s about 8 weeks. Once in a while he gives a light Baaaaaaeeeeee as a warning noise.
I had two cockerels for a while. One EE started crowing at 16 weeks, and the other (a BO roo) crowed at 17 weeks, the day after I gave him away to a flock without a roo. So I think it's individual to the roo and situational as well.
It will be dependant on the bravery of a little roo, I have had some that born and raised together the dominant one started just past a month, and others in with him went a couple months before attempting to crow. I have some together now, that only one does the crowing in the coop, but when let out the younger ones will scramble out the door, and when they feel far enough away from the head roo wiill then crow.