I'd say the earlier the better - one of my bm's received the wrong dress and since i ordered early, we had plenty of time to rectify.
I'd say start looking at least 8 months out and order around 6 months out. Unless you're going through an online vendor. You probably don't have to plan so far out -but thats an assumption on my part.
Stores/Vendors will give you specifics on certain dresses/designers when you look though.