Aart's right, we need breeders to answer this specific question. I used plural "breeders" because I expect different people use different time lengths.
I remember a thread probably 7 or 8 years ago where breeders were talking about keeping pairs, trios, or quads. I lost the link to it when I had a computer crash. What they pointed out was that they do not use cockerels and pullets, they use roosters and hens when they are breeding for show in pairs or trios. Again I'm sure different breeders do different things but at least to the few breeders on that thread the maturity of the chickens was an important variable.
