I have no previous experience of wedding dress ordering obviously but I've heard they take around 6 months, yours has obviously taken a bit longer though. I do vaguely remember seeing somewhere that in some cases they can take up to 9 months, but was under the impression this was rare.
I think from speaking to shop assistants it varies from designer to designer - for instance White Rose, who design my dress, are apparently fairly quick and only take between 4 and 6 months tops, but then others take much longer. I wouldn't worry about it, maybe just give them a call and see if they've had any news, for peace of mind if nothing else?