Our show calves get weaned around 6 - 8 months, when the rest of the calves get weaned. But if it is a particularly good calf, especially a bull, or a calf at foot on a show cow, we will leave them on longer, till they are 10 - 11 months or however long the cow can handle them. The longer they stay on milk the better.
Usually ours aren't broken in till a good time after weaning, 10 - 12 months, but many people like to get a halter on them as early as possible.