The most popular question we are asked here at Just a day is about timing. How long do I need to purchase a Wedding Gown? Well! the answer I would always give, "it depends on you".
If you have the time, then give yourself plenty. On average a gown will take around 5 months to come from the manufacturer.
Then you will need time for your alterations and fittings. So, quite simply start early, you will give yourself more choice and you will probably enjoy your experience a whole lot more.