Most pros say to order the wedding dress 6 months in advance and I would think it would be about the same for the bridesmaids dresses. You should begin right away to avoid any problems and stress.
I would say that it will be best to order the bridesmaids dresses 4-5 months ahead if buying from a store but if you are having them made you should start that at least 6 moths before time because they have to get measured and everything.