I'm not sure how far out you can reserve, but you have to call them directly... it doesn't go through the regular wdw-dine line. I think they can give you the number when you call the wdw-dine line, or you can just google it I'm sure.
I cannot remember how far in advance I called for reservations before. I did have to call them directly though. This trip we are going Rainforest Cafe in AK and I made the reservations this morning, 180 days out.
I called two weeks ago to make a Priority Seating Reservation at the Rainforest Cafe for May 1st. There wasn't any problem the person said "Yea no problem" and it only took like 5 minutes (from start to finish)
Yes, but it has to be exactly 180 days or later. My 180 day for our arrival date of our trip was Monday. Since I was staying on Disney property, I was able to make all of my ADRs for all of my trip days (180 plus my other 7). However, AK was the first park day I had scheduled; therefore it was the day after our arrival date. I was NOT able to book Rainforest Cafe at AK Monday. Since it had to be the exact 180 day, I had to call and schedule it this morning. So you can call WDW dining and book it no earlier than 180 days in advance.
I have been able to make my reservations for early October, was too early to make them for someone wanting 11 October though. This was for the DTD location, so direct call to them, not through disney dining.