For a Custom Wedding, you need to give at least 1 Month before the event date. For an Intimate Wedding, it is 1 week. Though depending on the time of year, you may find it difficult to get the locations you want at that short a time.
Disney Weddings are not something that can be really planned far in advance, just because of the way Disney works. When you call, you get assigned a Sales Manager, who helps you with the initial planning and can answer a lot of your questions. At the one year mark, Disney will place your request for date, times, and locations into the system. This will tentatively hold the date for you.
At the 8 Month Mark is when you can first sign your contract with Disney. If more than one person wants the same date/time/location, Disney will do a lottery to see who gets the date/time/location. I have heard a few brides who had to do this on really popular times of the year, but I had no time securing my date/time/location - even when I need to move my date five and a half months prior to my wedding date. Once you sign your contract and pay your deposit, your date/time/location is secured.
At the 6 month mark, your prices are guaranteed.