I can totally understand how frustrating that can be. But a big thing is it depends on where you order it from. Another thing I was told, but like I said it depends on where you order it from and the designer. The designers KNOW when they need to get you your dress, thats why you give them your wedding date. Alot of girls ordered Maggie dress over 12 months prior to their weddings and they didnt get their dress until 9+ months after....my dress was ordered the last week in sept and It came in last week. And it was customized...I wouldnt worry too much but I would definitely give them a date of at least 2 weeks prior.
GOod Luck Millie, I hope you get to do this really soon!