As a rule you can't book table reservations more than 60 days out. For activities whose availability varies based on the park schedule (Fantasmic and World of Color dining are prime examples) reservations sometimes don't become available for booking until well within the 60 day window (i.e. at 50 or even 45 days out).
The one thing that really needs to be booked as early as possible is Fantasmic dining reservations at the Blue Bayou. If this is something you're interested in you should start looking at 60 days and plan on continue checking every day or so until they become available.
Most other reservations aren't nearly as difficult to obtain. In general earlier is better, but you can still get reservations at 2-3 weeks out in most cases.