My RIR has been crowing since about 5-6 weeks.....it became "real crowing" pretty quickly and he crows OFTEN. He is also not a very nice boy. I think his hormones are just craaaaazy.
My Jersey Giant cockerel is 13 weeks old and I have only heard him crow once....while working outside, kept hearing a weird honking croak coming from their pen, so I snuck around the corner and caught him mid-crow. That was only a week ago and I haven't heard anything since. He might be crowing but I don't hear it (of course, i'm gone at work most of the day and he is down the hill back behind the garage which may muffle the sound). He is a nice boy so far.
Someone once posted that the ones who crow a lot...generally have something to crow about. I have found that to be true. My RIR is needing a new home because of his attitude. Very protective of his girl though.....I will miss his crowing....