We had a custom wedding, and we gave Disney all of our final numbers a month and a half in advance. This way they could get the final budget together and we paid one month in advance. Before this I had an idea of how many we would have so I let them know, but of course it was different. We sent our invitations out a litle early. We got married on March 26, and sent our invites out the 2nd week of January. Gave them a little over one month to respond. That was perfect for us, and it gave us time to figure everything out. Because you pay Disney one month out from the wedding, make sure to have your invitations in your hands a little time before that to get your numbers together.
Hope this helps
Jamie