It really depends, but if I had to guess, 14 weeks. But they still tweet quite a lot like the chicks, but calm down at 18 weeks and only speak when necessary.
My SILKIES have been in between both types of communication. It is funny some of them do it yet others not so much. When mine get excited usually about food they forget that they are 11 weeks old and go back and forth ,both sounds. Some at first looked like"did I do that?"I doubt my chicks will quiet unless they sleep and even then there is pillow talk, mumbles and a jolly good time had by all...................
My EE pullets are 12 weeks now and they are making almost all adult noises. Every once in a while I hear a babyish sound still. I think they were about 8 weeks when they started making adult sounds once in a while.