I think there is considerable variation in the timing of the first crow. Only 2 of my roos have crowed yet. The first was a buff bantam brahma and he started at 12 weeks. I rehomed him, and a dark bantam brahma roo who was the low man on the totem pole. The poor little dark roo never said a thing - too frightened by the loud guy, I think.
Within a week of getting rid of the buff roo, my 8 week old LF Welsummer started crowing. I have a 12 week old Ameraucana that I am virtually certain is a roo, and I haven't heard a peep out of him. Perhaps he is just cowed by the Welsummer roo.
So - if you haven't gotten a crow yet - you might at any time! I think my roos were spurred on by neighbors. We can hear, faintly, a rooster across the river to the north, and another rooster to the west. I think my fellows may have been calling out to those guys - saying hello or saying stay off my turf!
By the way - both of my roos' first efforts were rather pathetic, almost painful to hear. They both practiced and got better. My Wellie is still sounding pretty weak after several weeks of effort. I think he needs to get his full growth before he sounds like a proper rooster.