I make mine 180 days before I check in. That is when you are allowed to start making them.
It is very important to do this because the good places get booked up fast. On my 180 days out, I called at 7am (when they open) and had to wait on hold over an hour. Many of the places I wanted already had some time slots booked and I couldn't get the time I wanted.
With the dining plan and deluxe dining plan, the restaurants are getting booked very early so if your 180 days have already passed, you should get on the phone asap and make your ADR's.