I believe 180 - but not like the dining 180 where at 180 days out you can book for the whole stay.
If you know the day you want to do it, I believe you have to be at least 180 days out from that. I tried to book our Twilight Safari at our 180 day mark, but I couldn't do it because the day I wanted was 185 days from our arrival. (Had to wait that extra couple of days.) I'd book as soon as you know you're certain. You can always cancel which is much easier than trying to book at the last minute.