Disney now allows booking ADR's at 180 days before arrival. It had been 90 days in advance until about a month or so ago. I don't think the majority of folks know about the change yet. But when it was 90 days, many folks called at exactly 7AM EST on that morning to book the most popular restaurants for their trip. Princess Breakfast at the Castle almost always sold out within a few hours at the 90 day mark.
Other popular restaurants - Le Cellier in Canada and Chef Mickey's at the CR come to mind - also fill quickly. For our October trip I called at 90 days and although I got every restaurant I wanted for the week, I did have to adjust some of the times I wanted because they were already not available.
The best advice is to book just as soon as you know where you want to eat, keeping in mind the 180 day window and the availablity of popular restaurants and all of the character meals which fill quickly.
Many folks also post about the fact that cancellations do occur and that they have had luck at getting last minute ADR's. It does depend on the business of the season you are going.
My best advice is if there is somewhere you have your heart set on eating at, book just as soon as you can.
Hope this helps.